What's a Steam account?

Ive been people saying there selling their steam accounts and i was just woundering what it is. If you can tell me that would be great. Thanks

 

Steam is sort of a website/application which allows you to purchase PC games and download them straight to your computer. Your account is what controls this and allows you to play with and talk with others and basically be with the steam community.

More market related, the accounts can be sold and basically all the games that have been purchased onto the account come with it.

The account can also be more or less valuable depending on the amount of digits there for the account number. (I know this from stuff I've seen around. I may be wrong.)