802.11a - 54Mbps 5.6ghz.
Pros - high performance, 5.6ghz means less interference from 2.4ghz stuff (other 2.4ghz networks, phones, microwaves, etc). Fewer people have 'a' so tends to be more secure.
Cons - higher frequency means shorter distances. more expensive hardware. very difficult to find 'a' products that support the Mac. 'a' not compatible with 'b' or 'g'.
802.11b (called Airport in Apple parlance) - 11Mbps 2.4ghz
Pros - most common, decent coverage (can do an entire house plus if the conditions are right). Excellent support for the Mac assuming Apple Airport products. Lots of choices in access points. Inexpensive
Cons - 2.4ghz means lots of potential interference (effects range from slowdown to outright failure). Very popular with war drivers so more potential attackers. Poor performance, fine for surfing, sucks for large file copying, streaming high res video (e.g. dvd/mpeg2), around 2mbps "real" throughput.
802.11g (Airport Extreme in Apple parlance) - 54Mbps 2.4ghz
Pros - higher speed than 'b' but better range than 'a' (about the same as 'b'). Compatability with 'b' clients ('b' and 'g' can exist on the same 'g' access point.
Cons - 2.4ghz means similar problems with interference. More expensive than 'b'.
I'm sure I've missed a few things, but that should cover all the major points. Fear not, choosing is fairly simple.
Avoid 'a'. It's fast, but usually only corp users who need the performance use it because of the lack of range and higher equipment costs. Very little support for the Mac as well.
Use 'g'(Airport Extreme) if you can afford it, 'b' if you can't. Important to note that 'b' and 'g' clients will work with a 'g' access point, so you can always get a 'g' AP and 'b' client cards to save some money, and then upgrade clients along the way. Another important thing to keep in mind is that you can use almost ANYONES 'b' access point with Airport. This should theoretically be true for 'g' as well, though there have been some reported problems. The IEEE has only last week released their "final" 'g' standard, so interop problems will hopefully go away once everyone upgrades their firmware.