Today's digital cameras store images on removable memory cards (a few cameras have small amounts of built-in memory, but even they accept memory cards). There are several types of memory cards on the market, and within each type are several varieties. Here's how to make sense of it all.
The first concern regarding memory cards is what type your camera takes. You can learn this from your camera manufacturer's website, the instruction manual or the store where you bought the camera.
There are a number of memory cards currently in use. All do their job (storing images) very well; your choice of type is dictated by your camera. Here's a brief rundown.
CompactFlash Cards (CF). The largest physically of the current memory cards (though still tiny at approximately 1.4x1.7 inches), CompactFlash Cards come in capacities from 128 MB to 8 GB. There are two varieties: Type I and the slightly thicker Type II. Most digital SLRs accept both varieties, but some compact digital cameras that use CF cards accept only Type I. Check it out before buying cards for your camera.
SmartMedia Cards (SM). Smaller than CF cards, SM cards were popular a few years back, but have been replaced by the even smaller SD and xD cards in newer digital cameras.
Secure Digital Cards (SD). Very small (0.94x1.26 inches), SD cards come in capacities from 128 MB to 2 GB and are used by many compact digital cameras as well as some D-SLRs. One nice feature of SD cards is built-in write protection: slide a tab, and the card is protected from overwriting your images (make sure the tab is in the "write" position when you insert the card to shoot). They're similar to MultiMediaCards (MMC), which can be used in some SD-card cameras.
xD-Picture Cards. Even tinier than SD cards, xD-Picture Cards come in capacities from 16 MB to 1 GB and are used mainly in cameras by Fujifilm and Olympus, the format's creators.
Memory Sticks (MS). Memory Sticks resemble a stick of chewing gum and come in capacities from 128 MB to 1 GB. They're used mainly by Sony digital cameras.
Microdrives are tiny hard drives that resemble Type II CF cards and can be used in most cameras that accept Type II CF cards. Unlike CF cards, Microdrives do contain moving parts.
How much capacity do you need? That depends on the size of the image files you shoot and how much shooting you want to do between card changes. If you shoot with an 8-megapixel D-SLR in RAW mode, you'll get about 110 shots on a 1 GB card. If you shoot with a 3-megapixel compact digital camera in "medium" quality JPEG mode, you'll get around 1,060 shots on the same 1 GB card. If you shoot large image files or don't want to change cards frequently, get larger-capacity cards—1 GB or greater.
Note that not all digital cameras can handle the highest-capacity memory cards. Cards above 2 GB use a file system called FAT32, and many digital cameras use FAT16. Fortunately, most of the highest-resolution digital cameras—the ones that would benefit most from high-capacity cards—do support FAT32 (to varying degrees), but check to be sure that yours does before buying a 4 GB or larger-capacity card.
When you use your memory card, you'll find that its capacity is actually a little less than the one marked on it. That's partly because a small portion of the card's capacity is used for formatting and partly because disk drive and memory card manufacturers define one megabyte as one million bytes, while computer manufacturers define one megabyte as 1,024 kilobytes (1,024,000 bytes). Don't panic; this is normal.
Memory cards come in different speeds. Obviously, faster ones will let you shoot faster and will download images faster, assuming your camera and card reader are equal to the task.
Card speeds are generally rated in "x": 40x, 133x, etc. Each x is a minimum sustained transfer rate of 150 kilobytes per second, so 40x is 6 megabytes per second and 133x is 20 MBps. If your camera shoots 3.6 MB images (as mine does in highest-quality JPEG mode), a 40x card would transfer 1.67 images per second and a 133x card 5.5 images per second. This assumes my camera can operate at those rates (my newer D-SLR can; my older one can't). Faster cards will also download images faster, assuming your computer and card reader can handle those speeds.
Before using a memory card in your camera, you must format it. Each camera has a formatting feature, and for best results, you should use it. That said, when testing dozens of different digital cameras over the years, I just pull the card out of one camera and stick it in the next and have never encountered a problem. The new camera just creates a new folder on the card and stores its images there.
Never remove a memory card while the camera is writing to it. You'll ruin the images on the card and possibly the card itself. To be safe, always wait for the camera's "writing" light to go out, then switch the camera off before removing a memory card. It's also best to have the camera switched off when inserting a card.
When a card is inserted into a camera, the camera's frame counter shows the number of shots remaining on the card. This figure is based on the succeeding images being identical to the last one shot. More complex scenes (such as intricate patterns of branches and leaves) result in larger image files than simpler scenes (such as a silhouetted boulder against a plain sky), especially when shooting compressed JPEG images. Thus, you might find that the counter behaves a little oddly at times. For example, if you shoot a series of complex scenes, then look at the frames remaining and shoot a simple scene, the frames remaining might actually be higher than before you took the last shot.
While you can plug your camera into your computer and download images directly, most photographers prefer using a card reader. This is a little device that plugs into the computer's USB or FireWire outlet and has slots for memory cards. There are readers for each type of card and multi-slot readers that can read a number of different cards. If you use only one card type, a reader for that type will suffice; if your camera(s) use more than one card type, get a reader that reads all of them.
FireWire and USB 2.0 readers download images noticeably faster than USB 1.1 readers, which is important when you're dealing with high-capacity cards. My FireWire reader will download a 2 GB card in a couple of minutes, while my USB 1.1 reader takes more than eight minutes.
If you downloaded the software that came with your camera, it will probably launch when you insert a card in your card reader. You can download the images using that software or with other image-management software, such as ACDSee. You can also just create a new folder on your desktop, label it with the camera used and date (30D-07/17/06, for example), and drag and drop the image folder(s) from the reader to the folder, although this makes it more difficult to find a particular image later.
Memory cards contain no moving parts, so they're pretty rugged. But they aren't waterproof, and heat and strong magnetic fields can damage them and any images they contain. Store them in a cool, dry place, preferably in the dustproof plastic holders they come with. I once accidentally laundered a CF card in its plastic case when I forgot to remove it from a pants pocket, and the card and images on it did survive the washer and the dryer. But I don't recommend this treatment!