at the end of Season 3 Buffy put her in a coma. When she emerged from the coma in Season 4 after a showdown with Buffy she left for LA. Buffy followed her there and basically ordered her to go to prison which she did. She broke out of prison in Buffy Season 7/Angel Season 4 to help Angel then went to Sunnydale to help Buffy. That's the short version.
posted over a year ago
Thought buffy ordered her to go to prison Faith went on her own accord thanks to early intervention from Angel and his help