Humans are relative newcomers on the galactic scene, having only recently discovered the technology for space travel and colonization. Upon the discovery of alien life, The Alliance was formed to defend human interests in space. Humans are considered inferior by the older races of the Council, who govern civilized space from a station called The Citadel. At the beginning of Mass Effect, humans are excluded from many political and military roles. Commander Shepard changes humanity's role in the Council over the course of the game, for better or worse.
Council race: Yes
Hated by: everybody, except possibly the Asari and Salarians
-Shepard (that's you!)
As the first human Spectre (essentially the Council's C.I.A.,) it fell to you to prove humans worthy of respect among the Council races. Shepard was given command of the Normandy and sent out to right the galaxy's many wrongs.
Kaidan was Shepard's first recruit. He's an Alliance marine with biotic implants and a possible love interest, if you played a female Shepard. He will appear in Mass Effect 2 (if he survived).
Ashley is an Alliance soldier who joined Shepard's team early on. She was a possible love interest if you played a male Shepard, and will appear in Mass Effect 2 (if she survived.)
-Captain David Anderson
Captain Anderson is one of most decorated humans in the Alliance military, and was the original captain of the Normandy. After the attack on the Citadel and the destruction of Sovereign, he took on a more political role.
-Jeff "Joker" Moreau
Joker is the wisecracking pilot of the Normandy.
Udina is a self-serving bureuacrat struggling to raise humanity's profile in the political structure of The Citadel. He frequently chided Shepard for making humanity look bad in front of the Council, right up until Shepard saved the Citadel from the Reaper/Geth attack.
Cerberus is a rabidly pro-human splinter group that operates outside of Alliance jurisdiction. It is often accused of being a terrorist organization for its brutal "anything goes" tactics. Cerberus was the focus of several side-quests in the first Mass Effect, in which Shepard destroyed research stations where inhumane experiments were being carried out.
The Asari are a seductive, blue skinned race that have strong biotic powers and a lifespan of over 1,000 years. All Asari are female, and are capable of mating and reproducing with either gender of any race. They are generally a peaceful and diplomatic people, though Asari Commandos are renowned for their ferocity.
Council race: Yes
Hated by: nobody - you've seen them dance, right?
Liara was an archaeologist who spent most of her life studying the Protheans until the Geth attacked her dig site looking for info on the Reapers. Shepard rescued her and persuaded her to join the crew of the Normandy. Liara was a possible love interest for either gender of Shepard, and appears in Mass Effect 2.
Matriarch Benezia was Liara's mother, and was a highly regarded spiritual leader among the Asari until she was mentally enslaved by the Reaper Sovereign and forced to become Saren's second in command. Shepard had to kill her after she couldn't break free of Sovereign's mind control.