The History Of GSR
In episode #2 of the first season, "Cool Change", Gil Grissom turns to greet the young CSI from San Francisco. He has called her to investigate the shooting of new CSI, Holly Gribbs. Holly later dies and Sara stays in Vegas, and with Grissom.
Although their relationship started at a forensic lecture in San Francisco, their love wasn't an easy one.
Grissom fought his feelings, while Sara let hers be known from the start.
"You okay?"
"Ninety-five."
"Excuse me?"
"Normally my pulse is seventy. When it gets to 95, I realize how mad I am. I-I have ten people working around the clock on this thing."
"You're too hard on yourself."
"No, no. I'm not mad at me. There's a body in there and that guy knows where it is!"
"What's your pulse at now?"
"You want to take a walk around the block? Get some air?"
"No."
"Clear your head ..."
"I'm fine."
"Okay."
[Sara reaches up, touches Grissom's face. Grissom is not sure how to take this.]
"Chalk ... from plaster."
"Oh."
[Grissom wipes his cheek and looks at the back of his hand. Then back at Sara.]
"Better go wash up."
[Sara goes back into the appartment, leaving Gil Grissom behind to look at her as she goes.]
-- Sara Sidle and Gil Grissom (Scuba Doobie-Doo)
In "You've Got Male", Sara realizes that she is alot like one of the victims. She has alot of take out menus, emails, and catalogs. She also gets to read an email from a vic to a suspect with Grissom that hits close to home:

"It's easy to wear your heart on your sleeve when you're not looking in his eyes."
-- Sara Sidle to Gil Grissom (You've Got Male)
In "The Finger", Sara is on a date with Hank, when Catherine comes into the diner with a finger of a kidnapping victim and leaves it with Sara.

"I don't normally date women from work but there was ... something about you. And it definitely wasn't your perfume."
"I cannot believe you brought that up."
-- Hank Peddigrew and Sara Sidle (The Finger)
"Burden Of Proof" gave us some insight into the Sara/Grissom relationship.
They worked on an experiment together. It involved raw meat.

"So, take some photos of the experiment for the D.A. And then ... uh ... get rid of that stuff."
"That meat... the raw meat... me?"
"Yeah."
"How many meals have we shared together?"
"I don't know."
"Take a guess -- over a year working together."
"Thirty."
"I'm a vegetarian. Everyone here knows I'm a vegetarian. I haven't eaten meat since we stayed up that night with that dead pig. It pains me to see ground beef; forget about cleaning it up."
"Okay. Have Nick do it."
-- Gil Grissom and Sara Sidle (Burden of Proof)

"What is this?"
"It's, uh, just what it says: It's a request for a leave of absence -- six months ... year, maybe."
"Why?"
"I was thinking of checking out the federal government system - FBI ..."
"We have the best lab in the country."
"I need a different work environment."
"What does that mean?"
"One with, um, communication ... respect."
"Everybody here respects you."
"You don't."
"Is this about that hamburger thing?"
"No, Grissom ... this is not about that Hamburger thing. I-I-I don't believe you. How can you reduce everything that I've said to some kind of single quirk? Do you think the problem here is just about me?"
"If you don't sign my leave, I'm going to have to quit."
"Hey, Sara?"
"The Lab needs you here."
"Great."
-- Gil Grissom and Sara Sidle (Burden of Proof)
Later, being a good friend, Catherine tries to help Grissom:
"I heard about you and ... uh ... Sara."
"Sara, you know, she gets very emotional ... "
"Are you in denial? No, that's ... no, no ... way too analytical. Wow, you got burned bad, huh? Welcome to the club. I got third-degree burns from my marriage. Happens to everybody. Everybody just moves on."
"Good. Let's move on."
"But you have to deal with it. You have to deal with it first. You got to deal with it before it goes away. You are the supervisor. You have responsibilities, and people are making a family around you whether you like it not, whether you give them permission or not. We don't have to go to the Grand Tetons together, just ... every now and then you got to lift your head up out of that microscope."
-- Catherine Willows and Gil Grissom (Burden of Proof)