Flash cards come in all different sizes, but they come in different speeds as well. Nowadays you’re most likely to end up with an SD or CompactFlash card. The speed of your flash card is important because most cameras nowadays are very fast. You can take many images in rapid succession, but if your card has a slow write speed it can’t keep up. For SD cards you’ll be best served by a Class 6 card. For CompactFlash, a card rated at 133x should do just fine.
Many DSLRs and compact cameras come with video capabilities, and writing this kind of data requires a fast flash card. Class 6 SD cards will still be enough for most point-and-shoots, but if your video-capable DSLR uses SD cards you’ll probably want a Class 10. Class 10 cards are not all created equal, however, and some are marginally faster than Class 6. In most cases any Class 10 should sufficient, and anything with a max write speed of 15MB per second be more than enough. Of course, it doesn’t hurt to get a faster card and some Class 10 SD cards are capable of write speeds twice that fast. CompactFlash cards are often used in higher-end DSLRs because they’re capable of faster speeds at a lower cost (mainly because they’re physically larger and that’s easier to achieve thanks to their size). A CompactFlash card rated 233x or higher should handle video in most any DSLR just fine, but faster cards will definitely make things run more smoothly.