What does RJ-45 mean?
Many people use the term "RJ-45's" to refer to Cat 5e jacks or Cat 5e plugs. This is not, however, always the case.
RJ-45 jacks and plugs have 8 pins. If you have a computer patch cable laying around, you can see them on the plug. (Those shiny metal lines on the end.) RJ-45's are sometimes referred to as 8P8C connectors. This stands for 8 Position (which describes the width) 8 Conductor (or 8 Connector depending on who you talk to).
There are many different types of RJ-45 jacks available including Cat 3, Cat 5e, Cat 6 and USOC (regular telephone). Usually your supplier will know which one you're asking for based on the type of wire you've purchased, but it doesn't hurt to check. Make sure that when you ask for RJ-45's, you get what you want by indicating the specification that you are trying to fulfill.
UPDATE: Thomas of TECHtionary 2.0 points out that some people may be curious as to what the "RJ" of RJ-45 stands for. It's short for "Registered Jack." Check out his site; it's definitely worth a look.