Media Variations
While there is no one standard for digital media, there are fortunately relatively few choices. The main rewritable media are as follows:

Comfact Flash
The post popular of memory formats, compact flash cards are amongst the cheapest/megabyte form of storage. They range from a scant few megabytes to 16 gig. A solid state device they are very reliable. (Please note that not all cameras support compact flash cards with capacities greater than 2 Gig, however most new cameras do.) Compact flash cards are also the most durable media. While I wouldn't try it, we've heard of cards being run through the washing machine or driven over by a car and still worked when placed in the camera.

Microdrives
Microdrives seem appealing at first. They are usually the cheapest/megabtye form of storage. However they have major drawbacks. First and foremost, you can't use them at high altitude. They are rated at only 10,000 feet. After that their heads may crash. Also, from a purely unscientific survey, it appears more people complain that microdrives crash, even under normal contiditions, than their solid state counterparts.

Memory Sticks
Leave it to SONY to not learn from history. They created their own format, which offers little technological advantage and lots of disadvantages. ASide from the fact that mostly only SONY cameras use them, their capacities do not rival their compact flash equivalents. To exaserbate the issue, memory sticks tend to cost more as well.

Multimedia/SD
How small do you need a memory card? The multimedia/SD cards try to answer this. Personally, I feel memory cards are small enough. They're too easy to lose as is, so who needs them smaller? There is a gray line between Multimedia cards and SD cards, the latter of which adds security features, so I recommend checking you camera's manual for exact compatibility requirements. Be aware that many cameras do not support SD cards over 2 gig. This is a problem with most cameras manufacturered before 2006-2007.

SmartMedia
These cards are slowly being phased out. I recommend not purchasing a camera that requires it.

Are All Cards Created the Same?
Be aware the answer to this question is no, but the difference in cards may or may not make a difference in your camera. Cards are rated for speed. A faster card like the Lexar 80x line may not work any faster in a slower camera or computer reader. While my camera and home desktop are lightening fast, my laptop has an incredibly slow built in reader. Even though I have a 40x card, it take almost 45 minutes to download a full 2 Gig card on a USB 1 reader. A similar download on my three year old desktop, which takes advantage of the faster card, take approxiately 10 minutes. Now cards are as fast as 133x or even faster. They take considerably less time to download on a fast computer, but still do not show a considerable difference in the camera.

While we used to recommend only one or two brands, I have had luck with Lexar, SanDisk, Transcend, and Kingston. Personally, I have never had a card fail from any of these brands. If you read the online Photography forums you will hear of cases where cards fail, however, we have not seen reports that any of the major brands are more or less reliable than the others.