As boy/keano have said, on these boards it refers to Interactive Brokers, a brokerage company.
However, on many brokerage websites in the US you will see the term IB used, and in these cases it means 'introducing broker'. In other words, they act as an introducing broker to a larger brokerage company through which your transactions would clear.
And as Interactive Brokers is the largest retail brokerage they don't have introducing brokers so you can't be an IB to IB. :cheesy: