Ok well...
Mac Taylor is played by Gary Sinise and is well, the 'main man'! He is head of the NY crimelab. Mac was origianlly from Chicago and serverved for the US as a decorated Marine Corps Officer. He then joined the NYPD and while working his way up the ranks he found a spot in the crime lab. Mac believes in following the evidence, not trusting to intuition. He looks at a crime scene (and often the world) with Veneziano’s theory of quantum physics in mind - everything is connected. In Mac's mind, if he and his team can just figure out the connections, then they can solve the crime.
His wife, Claire died on 9/11 and most of season 1 (where mac's personal side is concerned) focused on this. Claire's son appeared in season 3 as he was adopted at a young age and wanted to find his mother. He is now dating one of the MEs, Dr Peyton Driscoll who first appeared in Season 3.
Stella Bonasera is played by Melina Kanakaredes. Stella is Mac's right hand woman. She began her career working as a patrol officer and once worked Narcotics out of Brooklyn North. She has a strong personality and her determination to solve a crime and help victims sometimes leads her to step on the toes of those who get in her way. She has at least four complaints on her record as a result. Stella is often referred to as the 'Statue of Liberty', due to her strong personality, determination and intelligence. Due to events in "Heart of Glass", it was suggested that she may be HIV-positive, but a test later revealed she is HIV-negative. Stella is a half-Greek, half-Italian orphan and her parents' identities are still unknown.
Danny Messer is played by Carmine Giovanazzo. Danny grew up on Staten Island, in an "Italian Mafia" family under surveillance. Very little is known about the Messer family but we do know that Danny has a brother, Louie. After graduating first in his class at the NYPD police academy, Mac personally selected Danny for his team, an honor and a responsibility that he attempts to live up to each day. While Danny was in college he played minor league baseball until he got into a fight and broke his wrist. For season 1 and part of 2 Danny was known as player but when Lindsay Monroe arrived in season 2 the writer's began to hint about a 'connection' that they share.
Lindsay Monroe is played by Anna Belknap. Lindsay is a native of Bozeman, Montana. Years before her arrival in New York, she was the lone survivor of a multiple homicide in which four of her friends were killed... the actual event is all 'revealed' in 318, Sleight out of hand. When she first arrived Danny teased her by calling her Montana, he did this because she had replaced Aiden Burns who he had a very good friendship with. This nickname became something special and after many bumps (because of her past) they seem to be in a relationship- in 324 they atre seen waking up together on a pool table in Danny's apartment.
Don(ald) Flack Jr is played by Eddie Cahill Flack is a homicide detective with the NYPD, known for both his wry wit and his determination to hunt down suspects. He is both a colleague and a friend to the members of the New York Crime Lab. His friendship with Mac Taylor is tested when Taylor uncovers evidence that one of Flack's men is a dirty cop and asks Don to turn over his memo notebook to prove it. Though Flack eventually capitulates, their relationship is a bit tense for a some time after. Flack is critically injured when a bomb goes off in a building he and Mac are evacuating. Until their rescue, Taylor is able to keep Flack alive using a shoelace and prior experience in the Beirut barracks bombing. Flack remains unconscious for the remainder of the episode, but has since fully recovered. When Flack returns to work, his friends tease him about using his scars to impress the ladies. His Father was also a NYPD cop and is mentioned every so often but we are still yet to see him!
Sheldon Hawkes is played by Hill Harper. Sheldon used to be a Me untill he became a CSI in season 2. He was a child prodigy who graduated college at eighteen, and by 24 he was a fully board-licensed surgeon.
In episode 304, Hung Out To Dry, Hawkes is targeted by killer Shane Casey. Several years before, when Hawkes was the ME, his testimony helped send Shane's brother, Ian, to jail for a robbery-homicide at a neighborhood pub. Ian Casey committed suicide while awaiting sentencing and Shane blamed several people, including Hawkes.
Others...
Adam Ross is played by A.J. Buckley. Adam a lab tech andwas injured (minor)in the season 3 final where himself and Danny were held hostage in a warehouse. He is not in the opening credits but I am glad that he will be reappearing this season.
Sid Hammerback is played by Robert Joy. Sid is also an ME and described as an "off-the-charts genius" who only recently traded in a career as a chef to become medical examiner. He has a habit of overtalking and oversharing, and has offered commentary on such unusual behaviors as threesomes and necrophilia which is known by most as 'that creepy place'!
Aiden Burn is played by Vanessa Ferlito. Aiden was a cast member in Season 1(+2). She was dismissed from the team in season two, episode 2 after becoming obsessed with a rape case concerning her friend Regina Bowen. Aiden pulled the evidence gathered from the first rape, which included one of Pratt's hairs. She broke the seal on the packet containing Pratt's hair and contemplated planting it at the alleged scene. Though Aiden did not actually plant the hair, the fact that she had broken the seal on the evidence packet earned her a severe reprimand from Mac.
After her dismissal from the crime lab, Aiden was apparently working on getting her private investigator's license while continuing to search for evidence that would conclusively tie Pratt to the Regina Bowen case. In the penultimate season 2 episode "Heroes," Aiden, after shadowing Pratt for months, was lured, ambushed, and brutally murdered by him, her body incinerated inside a stolen car. However, knowing that her time was drawing short, she left behind critical evidence that she knew would help her former colleagues to close in on Pratt and finally see him arrested. Aiden's murder devastated Mac and the rest of the team, particularly Detective Danny Messer. Though she at one time had joked that she was way out of his league, Aiden and Danny shared a very close, sibling-like relationship.
I hope that helps