Characters:
Shepard: Shepard is main character, the one that's playable, having saved the galaxy from the Reaper threat two years prior to the events of Mass Effect 2 Cerberus believe that Shepard is the only person capable of saving them from this new threat.
Jacob: An ex-Alliance military operative now working for Cerberus, has some biotic powers as well as technical abilities, is the first the squad mate met in the game, was fed up by how the Alliance operated so left to join Cerberus.
Miranda: Genetically engineered by her father, Miranda oversaw the Lazarus project
she ran away from her father who was trying to control her and thought of her only as a "legacy" and not a daughter, has strong biotic abilities.
Garrus: Garrus is a returning character from Mass Effect 1, with biotic abilities as well as prowess with a sniper rifle. In Mass Effect to begin with he is a officer with Citadel Security (or C-sec for short) who was fed up with the beaurocracy of the Citadel so joined Shepard's group to fight Saren. In this game he's found fighting three large mercenary. groups on the Omega space station under the guise of Archangel and agrees to join Shepard once again for the suicide mission.
Mordin: Mordin is a Salarian doctor who was a member of the salarian special forces in his youth, he's a biotic user. He too is found on Omega treating those affected by a plague in the poorer district of Omega, you must put a cure into the air to recruit him.
Grunt: Grunt is a tank bred "pure" Krogan created by the Krogan scientist (Krogan scientist, that doesn't sound right

) Okeer, on the planet Korlus, Shepard is originally sent to recruit Okeer himself but ends up taking Grunt with him/her after an "incident" with the head of the facility s/he infiltrated to find Okeer. Grunt doesn't have to be taken out of the tank and for the entirety of the game be left in the cargo hold but letting him out is a great idea as he's amazingly strong, using shotguns and assault rifles as well as ammo powers and and the concussive shot power (similar to the shotgun power from the first game, turning the shot into a missile of sorts)
Jack/Subject Zero: Experimented on as a child by Cerberus to be the most powerful biotic user ever, Jack has been turned into a psychotic sadist, revelling in killing her opponents, she's found in an orbital prison that Shepard finds his/herself a new member of, after the warden's betrayal. She is a very powerful biotics user.
Zaeed: One of the Galaxy's most infamous bounty hunters, Zaeed joins Shepard's crew (after the player has downloaded the DLC from the Cerberus Network) after being paid a large sum of credits by Cerberus. Similar to Grunt in abilities except for the type of ammo power and the loyalty power, Zaeed is the only character, other than the default squad of Jacob and Miranda, that requires no quest of any kind for him to join.
Thane: A Drell assassin who is dying from a terminal illness, he agrees to join Shepard's squad to find a purpose in life and atone for his past misdeeds. Thane has powerful close combat biotics and is skilled with a sniper rifle
Tali: Tali is a Quarian, the race that created the Geth (the main enemies of Mass Effect), who must remain in enviro-suits at all times due to their weak immune systems, she is a returning character from Mass Effect. Tali is a tech specialist and is best suited to fighting synthetic enemies.
Samara: Samara is an Asari Justicar, a highly revered order who uphold justice throughout the galaxy, usually using lethal force. She is a very powerful biotic user and, in my opinion, has more powerful and useful biotic powers.
Legion:The Collectors: The Collectors are the main antagonists of Mass Effect 2, they've been abducting human colonies
MAJOR SPOILER:The Illusive Man: The Illusive man is the head of Cerberus, he's the one that gives Shepard almost all the information used in the mission.