Take almost any portable digital device these days and you'll find a small slot in the side. They're on digital cameras, handheld computers, MP3 players and even digital camcorders.

These slots allow you to add extra memory, either to supplement what's there already or to fit some in a device that doesn't have any of their own.

The cards are used in just the same way as a floppy disk but with one important difference: they have no moving parts. The technical name for this is 'solid state' and it makes memory cards extremely reliable and very robust.

Memory cards also retain their contents even when they're removed from a device and those contents cannot easily be deleted. So your files are safe, even if the batteries in your MP3 player go flat or you accidentally bake your digital camera.

The other handy thing about memory cards is that they make it easy to transfer data from one device to another without having to connect wires and set up complex software. Just take your card out of the camera, pop it into your memory-card printer and press the button to create a glossy print you can frame and hang on the wall.

Unfortunately, technology being what it is, memory cards come in a confusing array of shapes and sizes. If you want to swap cards between your different gadgets, you have to choose carefully. Better still, read on as we demystify the world of memory cards.

Greetings, cards
Computer memory (or Ram, to give it its proper name) only holds information when it is powered. Remove the power and its contents disappear. Memory cards are different; once something is saved there it stays there, whether there's power or not.

This is because they use 'flash' memory, so-called because it cannot be erased without 'flashing' the chip with a belt of electric current.

The big drawback with flash memory is the price. Flash memory currently costs about £1 per megabyte on the high street, compared with around 30p per megabyte for a PC Dual In-line Memory Module. This means that a 1GB memory card is likely to cost an eye-watering £700.

Prices are coming down, however, and a 32MB card can now be bought for as little as £15. The trick here is to buy online; the different brands of memory card are much the same and web prices are nearly half those on the high street.

Digital photographers who need large amounts of high-speed memory should take a look at specialist photographic websites for advice.

CompactFlash
The granddaddy of memory cards, the CompactFlash card, is the size of a matchbook and a couple of millimetres thick.

CompactFlash cards come in two types. Type II cards are thicker than Type I, which means they can accommodate extra functions, and Type II hard disks and modems are available. Type II cards can only fit in Type II slots but these slots can use Type I cards too.

Smart Media
Smart Media is a thin square of plastic with gold contacts printed on the side. Frankly, Smart Media is a technology on the way out and it is not to be recommended. The contacts are vulnerable and the memory capacity is limited to 128MB.

Worse still, Smart Media-based devices made before 2001 may not be able to read cards bigger than 32MB because the required electronics are in the reader, not the card (unlike CompactFlash).

Memory Stick
Memory Sticks are the size of a stick of chewing gum and are a proprietary format used by Sony. No surprises then that any Sony device that uses solid state memory uses a Memory Stick, but you'll be hard pressed to find the slots anywhere else.

Memory Sticks are very robust and easy to use but tend to be more expensive per megabyte than CompactFlash and are limited to 128MB.

As well as the standard Memory Stick, a Magic Gate version also exists for use in Sony's digital music players. Magic Gate Sticks incorporate copyright protection and are even more expensive.

Predictably, the full-size Stick format is proving too big for many gadgets and Sony has already introduced a smaller version, the Memory Stick Duo. The Duo can be used in a regular Memory Stick slot with an adaptor, but not vice versa.

Multimedia card (MMC)
The MMC card is the emerging standard memory card for all portable devices. The size of a postage stamp, MMC cards are beginning to replace both Smart Media and CompactFlash in the latest cameras and handheld computers.

Unfortunately, the designers of the MMC did not predict the rise of MP3 players, which put the music industry on alert about the possibility of piracy. So, while many early devices use MMC, most new ones can only use its successor, the Secure Digital card.

Secure Digital (SD) card
SD cards have encryption built in so that music can be stored in such a way that it cannot be copied. Almost all new gadgets now have SD card slots rather than MMC, though they will read MMC cards as well (but not, as ever, vice versa).

SD cards feature a 'write protect' tab that instantly protects valuable data from being inadvertently erased.

They are rapidly gaining ground and will be the most common memory card format before long. The cards are robust and small enough to keep several in your wallet if needed. They are expensive at the moment but prices are bound to drop as they become more popular.

USB flash memory
Here's the problem: you have to take a large document home to do some more work on but it's too big to fit on a floppy disk.

The answer? A flash memory chip with a USB plug, so you can plug it in to your office computer, copy the file onto it, then pop it in your pocket for the journey home. Once there, you simply plug it into the USB socket of your home computer and access the file.

Several USB flash memory devices are now on the market and they are very easy to use. The disadvantage of USB flash compared with CD-R or even floppy disks is the high cost, but they are small enough to attach to your key ring and are difficult to damage.

The natty Diskonkey from M-Systems, also sold as the Disgo, is a lipstick-size object with a key ring on the cap.

Remove the cap and the USB plug is revealed. Plug it in and the computer automatically recognises it. Files are stored on it simply by dragging and dropping them onto the icon. It's as simple as that.

Prices start at around £35 for 8MB to a whopping £230 for the 256MB model. A 512MB model is due soon too, so start saving now if you want one.

The problem with the Disgo and its ilk is that the memory is fixed, so you can't reuse it in your camera or your handheld computer as you would with a card, which makes the new SanDisk Cruzer a must-have.

The Cruzer looks like a miniature hip flask with a slider in the middle. Slide the button one way and a USB plug comes out. Slide it the other and its MMC or SD card is ejected.

The cards can be swapped for higher capacity ones, making it a simple way to transfer your files from a digital camera, for example. The prices are keen too, at around £30 for a Cruzer with a 32MB card to about £120 for a 245MB model.

Is this your card?
If you have been acquiring digital gadgets for a few years now, you'll almost certainly have a number of flash cards of different types.

If you want to read them all on your PC, get a card reader. The best bet is the Dazzle Universal 6-in-1 reader from SCM Micro. You can plug CompactFlash, Microdrive, SD, MMC, Smart Media and Memory Stick cards into the device and it will be read. You can even read two cards at the same time.

It connects to a USB port so is simple to install and, at around £70, it's not unreasonable for a piece of kit that will connect the card from almost any portable device to your PC. The choice of card for you depends on its use; just don't forget that prices are falling and formats are changing.

Plugging in
Memory cards operate just how computer devices should: slide a card into its slot and it works. In cameras and other portable devices the card should seem invisible, storing the images, audio tracks or whatever onto the card as a matter of course.

When the card is in a reader connected to a PC's USB port, it should appear automatically as just another drive in My Computer; double-click on its icon and its contents will appear in a window.

Windows XP does this particularly smoothly but Windows 98 users may have to install special drivers supplied with the reader.

Mac users should have no difficulty installing a memory card reader either but, if you want to swap data between a Mac and a PC, you should format the card on the PC first. Macs can read PC disks but not vice versa.

It also helps to have the same applications running on both machines so the files can be read easily, otherwise you will be forced to find a common format that can be read by both (for example, plain text for word processors or bitmaps for image editing).

The future
Memory cards have a bright future. They are going to be everywhere: in cameras, music players, mobile phones and in all likelihood, your hi-fi and car as well.

SD and MMC cards will dominate, becoming the means of exchange for data on mobile devices. If you want to swap digital images with friends, simply take your MMC card out of your camera and pass it round so they can download the snap onto their mobile phones.

To buy an album to play on the way to work, just download a file from the record company's website onto your SD card and you're all set for the journey.

Sony's Memory Stick format will come a distant second, kept afloat by the massive cross-media clout of the electronics giant. The miniature version, Memory Stick Duo, will be fitted in Sony Ericsson's exciting P800 Smartphone when it arrives this winter.

Smart Media is already fading fast and is likely to disappear soon, although its backers, Toshiba and Fuji, are launching a new miniature version called XD Picture Card, which has about the same dimensions as SD.

CompactFlash is already declining as MMC and SD take off, but it is likely to survive because of the standard's versatility; enough to cram in hard disks, satellite navigation sensors, cameras and loads of other devices.

Contacts:

SanDisk
Lexar
Dane-elec
Sony
Kingston
Diskonkey
Dazzle Universal
Microsoft