The Krogan are native to the planet Tuchanka. After a series of nuclear wars spread dangerous radioactive fallout across the planet, the Krogan were forced to adapt to their harsh environments. As a result of generations of Krogans being forced to endure this environment, the species as a whole have become virtually impervious to toxins, radiation, and extreme temperature. They also store water and nutrients in a large hump on their back, much like a terrestrial camel. Their skin is extremely thick and tough, so bullets and explosions do minimal damage. They also have multiples of major organs just in case one gets damaged; both of these qualities make Krogan extremely hard to kill, and extremely well suited for combat. They quickly reproduce and mature in order to keep their population steady. Also, Krogans have four testicles.
The Krogan were an important part in defeating a dangerous race of sentient insects known as the Rachni. This insectoid species was unfortunately extremely hostile, and after a science team from Citadel Space inadvertently opened a mass relay to their home system, the Rachni launched massive invasion campaign. After decades of war the Rachni were turning victorious, until the Salarians introduced the Krogan into the equation. The Krogans' massive numbers, coupled with the species being notoriously tough to kill, enabled the Krogan to beat the Rachni back to their home system, and eventually hunted the Rachni to extinction. After the defeat of the Rachni, the Krogan were considered saviors of the galaxy, and were honored for their role in the Rachni wars. Unfortunately their quick reproduction cycle meant that they started to spread across the galaxy very quickly, colonizing many worlds until they started aggressively colonizing already occupied worlds through force, and into territories that they were not allowed to colonize by The Citadel. This led to hostilities between them and the citadel council leading to the Krogan Rebellion.
Due to their hardy nature and numbers they were successful against the joint attacks of the Citadel races. The Turians played a big role in keeping the Krogan at bay due to their military might but were still not able to bring some sort of conclusion to the war. In their desperation the Citadel races looked to the Salarians, who created a biological virus, called the genophage, that would infect almost all the Krogans and make breeding very difficult. The Turians unleashed the genophage which had with tremendous effect on the Krogans and brought their reproduction to a crawl. Only 1 in 1000 births were successful as the rest died in stillbirth. Not being able to maintain their numbers the Krogan were defeated and the Krogan Rebellion came to an end. It is widely believed that the Turians were the ones who made the decision to unleash the genophage but some people such as commander Shepard's Krogan ally Wrex believe that the Turians were not alone and that they had gained approval from the Citadel Council in doing so.