The number given to your Steam account
The 1st steam Account was 0:0:1, the second was 0:0:2 and so on.
The lower the STEAM Id the rarer the account is and the longer the person has been playing.
Leagues such as CAL
require you to enter your Steam ID so they can verify its really you playing.
"Wow dude that guy who joined STEAM Id was 0:1:20031934 he must be a really bad n00b!"