An escrow the one who holds the funds for the transaction , assuming you are selling a steam account , the buyer send the funds to the escrow , you(the seller) give the account to the buyer , If he confirms that everything on the account was as you described and It works fine , the escrow release the funds and send you the coins.
Actually that's the definition and though I appreciate that but actually Sylar wanted to know about how can he get started with escrow here, how it works here and what are the best guys for that.
In case Sylar you are yet not sure how to do it please send me a PM and I will try to arrange a genuine escrow for you.