Well, sizing can be really hard since every brand is a little different. You want the strap (if it has them) to only give you 10% of the support, the band should do the rest. Most women buy the wrong size, or have to actually get measured by someone to know what to buy.
There are many types. There's your standard, convertible, strapless, padded, water bra, bustier, body shapers, non-padded, full coverage, demi-cup, quarter cup, and the list goes on.
Certain bra sizes can be really hard to find, or you have to have them custom made. I have a fairly difficult size to find which sucks. If I get the boob job I have been wanting I'll have to get them custom made or maybe from a special order place.
I could go on, but I think I'll stop.