For example the SD form factor is the most popular, while the CF-II allows you to use Microdrives. Some media card types such as SmartMedia never took off.
Personally, one of my selection factors for buying new electronics is that the device needs to support SD cards. I hate having devices that can't support the same type of removable media. This is handy when you need to be able to off load the media to another device quickly.
Below is an excerpt from an Wikipedia article on the different type of media cards available. You might notice some media card form factors that you may never have heard of before, I know I did. If you want more information on any of them just click the link.
|SmartMedia||SM / SMC||None|
|Memory Stick Duo||MSD||MagicGate|
|Memory Stick Micro M2||M2||MagicGate|
|Reduced Size Multimedia Card||RS-MMC||None|
|Secure Digital Card||SD||CPRM|