Derek Shepherd is a renowned brain surgeon who we are led to believe moved to Seattle from New York a little bit before the series debuted. He and his intern, Meredith Grey, met and slept together during the Pilot, before they even knew each other's identity, with their romance blossoming soon after.
Derek is married to Dr. Addison Montgomery Shepherd, who he left in New York after finding her in bed with his best friend Mark Sloan, a plastic surgeon. Addison joining the staff at Seattle Grace during Season Two resulted in constant conflict for Derek as he struggles to repair his marriage and deal with his feelings for Ms. Grey. Ultimately, he goes for his true love.
Dr. Shepherd was nicknamed "McDreamy" by the female interns at Seattle Grace. The man has some amazing hair. He and Dr. Preston Burke (while he was around) were among the hospital's top surgeons and there has been occasional friction other which one the chief, Dr. Richard Webber, would name as his successor.
Derek was named Chief of Surgery by Richard at the end of Season Three, but he did not accept. He and Addison got a divorce after he rekindled his love for Mer, but he has expressed doubts over that relationship's chances of lasting, as well. As he put it, he doesn't know if he can keep breathing for her.
We'll see what Season Four holds for these two.
MEREDITH: "You don't get to call me a whore. When I met you, I thought I had found the person that I was going to spend the rest of my life with. I was done! All the boys and all the bars and all the obvious daddy issues, who cares? I was done. You left me. You chose Addison. I'm all glued back together now. I make no apologies for how I chose to repair what you broke. You don't get to call me a whore."
DEREK: "This thing with us is finished. It's over."
DEREK: "Yeah, it's done."
MEREDITH: "It is done."
Lexie: [narrating] Grief may be a thing we all have in common, but it looks different on everyone.
Mark: It isn't just death we have to grieve. It's life. It's loss. It's change.
Alex: And when we wonder why it has to suck so much sometimes, has to hurt so bad. The thing we gotta try to remember is that it can turn on a dime.
Izzie: That's how you stay alive. When it hurts so much you can't breathe, that's how you survive.
Derek: By remembering that one day, somehow, impossibly, you won't feel this way. It won't hurt this much.
Bailey: Grief comes in its own time for everyone, in its own way.
Owen: So the best we can do, the best anyone can do, is try for honesty.
Meredith: The really crappy thing, the very worst part of grief is that you can't control it.
Arizona: The best we can do is try to let ourselves feel it when it comes.
Callie: And let it go when we can.
Meredith: The very worst part is that the minute you think you're past it, it starts all over again.
Cristina: And always, every time, it takes your breath away.
Meredith: There are five stages of grief. They look different on all of us, but there are always five.