The crossroads Demon was first seen in the episode "Crossroads Blues" when Dean and Sam try to save the life of an innocent man who just wanted his wife to live past the date her doctors gave her.  This is also the episode where we learn about humans making deals with demons.  Each deal's contract is held by Lilith and the person asking for the deal is granted only ten years to live in return for the deed.  After ten years the person is hunted down by Hell Hounds and brought to hell to complete their deal.  For some reason the demon is always a woman and never a man.

This demon does not reappear until later on in Season one when we see Dean summoning her to make a deal for Sam's life.   She is to bring back Sam, as he has died in the episode, and Dean will go to hell willingly after 10 years.  Only she give Dean one year to live instead of ten and he reluctantly agrees to the deal.  

During Season three Sam summons the crossroad demon to "bully" her into releasing Dean from his contract with the colt.  She refuses because she does not hold the actual contract and he kills her with a bullet from the colt.