Why don't you just google it. There are tons of stupid questions on this site which can just be easily googled and I don't know why people don't do it. And isn't steam some program where you can buy games, like a store or something.
SteamID refers to the program Steam. It is a program which can be used to buy and download games for PC and then play them over the internet with a large community.
Steam is quite helpful in setting up matches with people and has quite a good User Interface. However, once you have downloaded a game, you cannot separate it from Steam, which can get a bit annoying. For example, I downloaded Track Mania Nations (for free) and wanted to give it to my friend who doesn't have the internet, and I couldn't.
[quote=Wikipedia]A SteamID is a unique identifier used to identify a single account on the Steam platform. It is also used to refer to a Steam user's Steam Community profile page.[/quote]