You say "deal" if you're agreeing to do something. Often, it's used during a compromise, where 2 people want different things, so they try to agree a balance.
eg. "I want to see The Hunger Games, but I can't afford to buy a ticket."
"Well, how about I buy your ticket, and then you buy the popcorn?"
".. okay, deal."
"I don't want to go running."
"Well, how about this: if you run, I'll give you a bar of chocolate as a reward."
"Haha okay, deal!"