Everything you need to know about Mass Effect

The Krogan are a warlike reptilian race who value strength and survival above all else. Their population has been decimated by a genophage, a virus that prevents normal reproduction. As their numbers dwindle, they become more depressed and violent while desperately searching for a cure.

Council race: No

Hated by: everybody, but especially the Turians

-Urdnot Wrex

A gun for hire and ruthless warrior, Wrex was one of the few remaining Krogan Battlemasters. He left the Krogan homeworld after a fight in which he killed his father. Wrex wanted to put a halt to petty tribal warring and focus on the threat of the genophage, while his father saw peace as a sign of weakness. If he survived, Wrex will appear in Mass Effect 2.

The Quarians are space gypsies, a nomadic people who live aboard the Migrant Fleet and are distrusted by Council races. The Quarians were driven from their homeworld by the Geth, artificial beings they created but couldn't control. Quarians have very delicate immune systems, and must wear full environmental suits at all times. As a rite of passage, every Quarian must complete a Pilgrimage and bring something useful back to the fleet before being fully accepted into Quarian society.

Council race: No

Hated by: everybody

-Tali'Zorah nar Rayya

While on her Pilgrimage, Tali uncovered evidence condemning the dodgy Spectre Saren. She gave the evidence to Shepard and joined the crew of the Normandy to continue her Pilgrimage. Like many Quarians, she's a techno-whiz and good at anything mechanical.

Salarians are expert scientists and run the galaxy's finest intelligence-gathering organization. They designed the Krogan genophage and were the second race, after the Asari, to discover The Citadel.

Council race: yes

Hated by: the Krogan

-Captain Kirrahe

Captain Kirrahe was the Salarian intelligence officer who uncovered Saren's facility on Virmire. He fought (and possibly died) alongside Shepard in the assault, which (possibly) cost Shepard as many as two crew members.

Turians are the mortal enemies of the Krogan, having fought Krogan expansionism for centuries at the behest of the Council. The Turians worked with the Salarians to spread the genophage that keeps the Krogan population supressed. Turian culture is rigid and stoic, with a strong emphasis on military service. Turians value order and stability, hence are easier to get along with than the Krogan.

Council race: yes

Hated by: humans, the Krogan

-Garrus Vakarian

Garrus was an officer with C-Sec, The Citadel's law enforcement organization. He's a hardboiled detective with military training who was investigating Saren prior to joining Shepard's team.

-Saren Arterius

Saren was a powerful Turian Spectre who was secretly working for the Reaper Sovereign. Because of his status as a Spectre, he was trusted by the Council until Shepard discredited him. Saren and Shepard then raced to discover The Conduit, an ancient Prothean artifact believed to be instrumental in resurrecting the Reapers. Shepard killed Saren in the final battle on The Citadel.