The Beginning - 2148
In the year 2148, a science team excavating Mars found the ruins of a civilization that was discivered much later on to be Prothean. The discovery of these ruins led to the advancement of human technology by hundreds of years. It wasn't long before the discovery of a Mass Relay. These immense extra terrestial artifacts act as catapaults capable of lauching vessels across almost indefinate reaches of space by punching a hole through the space-time continuum. In other words, faster than light (FTL) travel. Thus, humanity entered into a whole new status among other galactic species.

The First Contact War - 2157
Having attained this new ability to explore the galaxy at will, aggressive human colonization and expansion attracted the attention of the Turian race (photo). The Turian demanded they cease infringing upon territory claimed by other species. The humans, at his point, were somewhat naieve in thier understanding of just how late their entrace into the galactic community was. They refused, and hostilities began as the Turians defended and attempted to reclaim territories taken by humanity. The hostilities continue only for a few months before the Council, the ruling body of Citadel space - both of which the humans were unaware even existed - intervened.

Part of the Whole - 2165
By 2165 the Council recognizes humanity as a power in the galaxy with enough influence to contribute to the government. They are granted an embassy in the Citadel, a planet-sized space station that serves as political and economic heart of the galaxy.

The Reaping
(It would really be very much more beneficial to simply read the plot section of Wikipedia. It does a very good job of explaining the game's story. I pretty much would have just copied and pasted... The rest about the different races and such I will post, but if it catches your interest, feel free to explore the Wiki.

Bring Down the Sky
In the Asgard system in the Skyllian Verge cluster, a group of Batarian terrorists took control of an asteroid being rerouted to orbit the planet Terra Nova, one of the largest human colonies. The asteroid was being moved into its new orbit by fusion jets, but the batarians seized control of the facilities controlling the jets and set it on a collision course with Terra Nova, home to over 4 million colonists. Believing the humans robbed them of space they owned (not a fact acknowledged by the Council), their leader, Balak set the plan in motion not long after Saren's defeat. And who better to send to fix the problem than the one responsible for stopping Saren? Commander Shepard arrived in time to shut down the fusion jets and spare the colony, but the batarians have made thier intentions clear.