The Birth: Piotr Malkavich Rasputin grew up on a farm in what was once called the U.S.S.R. The first time he ever manifested his powers was when his baby sister Illianya was unsuspectingly in the way of a run away tractor. Piotr used his great strength and invunerability to save save his sister. The other peasants considered him a freak and tried to run him out of town. Xavier happened to be nearby and told Piotr that his powers belonged to the world. This inspired the young Russian; who having nowhere to go joined Professor Xavier's X-Men.

The Alter Ego: While Colossus became a legend, not much was mentioned about the man behind the legend - Piotr Rasputin. He once had a younger sister; Illianya and a brother; Mikhail who is also supposedly dead.
Hobby-wise, we know that Piotr likes to paint.

The Legend: Taking the name of Colossus, Piotr stepped into the shoes of the X-Men. He, Wolverine and a few others had been hastily recruited by Professor X to save the original team of X-Men from the mutant island; Krakoa. After Colossus saved the original team members, he stayed on with the team for quite a long time.
During the Morlock Massacre, Colossus couldn't change back into his human form, resulting in him almost dying. Just after that, his brother Mikhail transported the remaining Morlocks to an alternate world, and apparently died.
Colossus also helped the Professor X and others to recruit the young and confused Kitty Pryde who later became Sprite. Kitty was being hunted by the Hellfire Club diring that time so that they could recruit her.
And then a tragic loss. Illianya was the first victim of Stryfes Legacy virus, and died. And it's evidently Mikhails fault. snapped like a twig, and joined the newley returned Magneto and his Acolytes.
After cribbin' with his new homies on Avalon (a large space station), Holocaust destroyed the place in a feirce battle with Exodus. After his return to Earth, Colossus helped defeat Gene Nation, lead by Marrow, who was apparently killed. Colossus then joined his former friends Nightcrawler and Shadowcat in Excalibur. This was the time when the team thought that Xavier had been killed. This lasted only for a few months; though since the team disbanded and joined the other X-Men.

Colossus has the ability to change his flesh into organic steel. He has been known to withstand Wolverines adamantium claws. In this form, he can lift several tons and is impervious to harm.