Really big past season SPOILERS!
The Interns:
Izzy Stevens: Pretty good doctor but gets way too attached to her patients. Last season she fell in love with a heart transplant patient, Denny, and he asked her to marry him. After a seemingly successful transplant, a blot clot killed him in last year's season finale.
Alex Karev: He's a jerk and originally wasn't that great of a doctor. He's getting better but still makes people mad all the time. He really ticked off Addison and she makes him to her baby doctoring stuff now just to irritate him. He likes Izzy and they went out a few times, but he slept with a nurse when they were going out. They almost got back together but Denny showed up. They're still friends though. He's my favorite character.
George O'Malley: He's a bit of a wimp, but he getting a pretty good rep as a doctor. His big moment came when he did a unassisted procedure (I think it was a heart one) in an elevator when he and Alex got stuck in the elevator with a patient. Alex froze. Since then, Burke (the black docter with the hurt hand) took him on as a protege and even let George live with him when he moved out of Grey's house temporarily. George secretly loved Grey the first two seasons and made a move on her last season. They started to sleep together, but Grey flipped out in the middle of it and hurt George's feelings real bad. He's going out with that Callie Torres girl now, which isn't that great of a fit. I think she's just his inbetween girl while trying to get over Meredith.
Christina Yang: She's all about being a surgeon and isn't very tolerant with the patients, she always says what she thinks. She's involved with Burke and lives with him. She was pregnant with Burke's baby without him knowing until she collapsed during surgery and lost the baby. She was going to abort the baby anyway.
Meredith Grey: She's obviously the main star but isn't very likable. She sleeps around quite a bit. Izzy and George live with her in her mom's old house. She met Derek the night before the internship began and thought it was a one night stand but he turned out to be one of her bosses. The first season they dated in private then it came out. Then Addison, Derek's wife, showed up out of the blue from NY, they were separated. The second season, Derek and Addison tried to work things out, but the season ended with Derek and Grey doing it during a party. Meredith's mother is a really successful surgeon, but now has alheimer's diesease. At first Meredith kept it a secret from everyone, but then one day her mom showed up in the ER and the lid was off. She wasn't that great of a mother as it seemed she put work way ahead of her family. Meredith's dad has shown up a couple times and he has a whole new family with a daughter that Meredith didn't know anything about, but she found out when they had to come to the hospital. I don't think Meredith's dad knows that she found out about his family. She slept with George one night when she was feeling bad about herself and almost recked their friendship. George eventually forgave her and they are becoming friends again.
The Doctors:
Addison Montgomery-Shepard: Derek's wife. She cheated on him with his best friend Mark, the guy coming out of the shower at the end of the episode a couple weeks ago. She's one of the best infant doctors in the country and was hired on to stay at Seattle Grace after she originally came just to try and get Derek to move back to NY. She tried real hard to get Derek and her marriage back during season 2, and Derek tried too, but he eventually couldn't get over Grey.
Preston Burke: He's one of the top surgeons along with Derek. He's dating Christina and is pretty much an all around good guy. He got caught in a shooting crossfire last season and is having a hard time rehabbing his hand.
Derek Shepard: "McDreamy" came on to be a surgeon at Seattle Grace the same time all the interns did. He is suppose to be the top nuerilogical surgeon in the country. He met Meredith the night before work started and she turned out to be one of his interns. He left NY after Addison cheated on him and has been trying to decide between Addison and Meredith ever since she showed up in the season 1 finale. Looks like he's finally choses Meredith.
Richard Webber: He's the hospital chief. He actually had an affair with Meredith's mom back when they were just starting as surgeons. Meredith eventually found out about it. Webber's wife has always been on him to spend more time with his family and she has recently given up and wants him gone.
Miranda Bailey: She's the intern's resident and she is awesome. She's nicknamed the Nazi but she has lightened up quite a bit since having a kid last season. She's still tough, but she does seem to care for the interns now.
Other guys:
Finn: He's the vet that Meredith started dating in season 2 while Derek was trying to work things out with Addison. He's a good guy, but now is in competition with Derek for Meredith.
Nurse Olivia: She's a reoccuring nurse that George dated for a while. They broke up when she gave him cyphilus that she got from sleeping with Alex :y4: She seems pretty nice besides that.
Callie Torres: Showed up at the end of last season as George's girlfriend after the crazyness with Meredith. She's a doctor at Seattle Grace. She sets bones and does stuff with feet a lot.
Joe: He owns the bar everyone goes to and is gay.