In sports betting, arbitrage is simply betting both sides of the same contest for profit.  Its not always easy to find opportunities, but when you do it is the closest thing to risk-free gambling there is.  Gamblers like them so much, they have a nice nickname for them -- "arbs."  Some sports books don't like them so much.  If they decide that you are an arb player, they might stop giving you bonuses.  To ease the pain, they will call you a "professional."  This is also called "scalping."

Immediate Arbitrage

This is the rarest and most lucrative situation.  At the exact same moment in time, two different sports books offer the lines on the same contest that diverge enough that you can't possibly lose.  For example:

Sports Book 1:                    Sports Book 2:
Eagles   +2.5 +114              Eagles   +2.5 -110
Redskins -2.5 -105               Redskins -2.5 -110

In this case, you can bet the Eagles at +2.5 +114 and the Redskins -2.5 -110.  You bet $100 on the Eagles and $112 on the Redkins.  If the Eagles win, you collect $114 which covers the $112 you lost on the Redskins and if the Redskins win, you collect $102 which covers the $100 you bet on the Eagles.  Either way, you win!

I'll flesh this section out more later.  I'm tired now.