Gaff shows up and congratulates Deckard on a job well done. He says it's too bad that Rachael won't live, given that Deck is in love with her.
Deckard races to his apartment, discovering that Rachael is still alive. Gaff was there, and he left behind a silver origami unicorn, which shows that he decided not to kill her. (It also hints that he knew Deckard's dreams, which may indicate that these dreams were implants, and that Deckard is actually… a replicant!)
Deckard wakes Rachael. She says she loves and trusts him, and they depart, gathering their things and getting into the elevator to leave.
If you're watching certain cuts of the movie (like the Director's Cut or Final Cut), this where the movie ends. But in the original theatrical version, Deckard and Rachael drive into a scenic, natural environment, and Deckard reveals via voiceover that Rachael doesn't have a limited four-year lifespan; instead, she's a special replicant with the lifespan of a normal human. So it's a totally unexpected happily-ever-after for the couple.