The Birth: Cain Marko is the son of Kurt Marko and Sharon Xavier (widow of Charles Xavier's father). When Cain was serving in the military along with his step brother, Xavier, the two were seperated from their platoon, and ended up in some sort of temple. It was there Marko got the Crimson Gem of Cytorrak, and became the Juggernaut. The gem invested anyone who touched it with the power of the god Cyttorak, turning Marko into an unstoppable living engine with the ability to smash through any barrier, and making him immune to pain and injury.
This all happened in X-Men #12, 1963

The Legend: Cain is intensely jealous of his step-brother, Charles Xavier. This may be because Cain was sent to a military boarding school, something the younger Charles never had to go through; because Charles out-performed him academically; or just because Charles once forgot to pass the salt at dinner and Cain never lets go of a grudge.
Cain always picked on his step-brother because of this jealousy. This may the reason why Onslaught (creation of the evil from Xavier's and Magneto mind) first attacked Cain before anyone else.
After discovering the immense power he had received from the gem, Cain Marko took the name of Juggernaut - a fitting name seeing that most of the heores find him unstoppable.
Although he's most likely to be found trying to pound Xavier's pupils the X-men into the ground, Cain has also been seen bludgeoning the likes of Spider-Man, Spider-Woman and Venom. He once almost killed Madame Web when he yanked her out of her life support system. His friend, Black Tom Cassidy, thought that Madame Web would make an excellent partner in his life of crime and asked Juggy to fetch her. That's when Madame Web telekinetically told Spider-Man that she is in danger and it was the first time that both Web and Spidey faced the likes of the Juggernaut.
Juggy has also faced the Hulk several times. He and fellow assassin-for-hire Deadpool have butted heads numerous times, a rivaly that is likely to continue since Deadpool is the type who holds grudges.
Cain is utterly devoted to his best friend, Black Tom Cassidy, even once trying to share the power of the Ruby of Cyttorak with Tom, though that didn't quite work out since Tom didn't like being so powerful. Tom's relative, Siryn of X-Force, still has a soft spot for ol' Juggy, despite feeling the need to try and knock his block off every now and again. Fortunately, Cain doesn't take that sort of thing too personally. Juggernaut also got along fairly well with his Exiles comrades in the Ultraverse before finding his way back to the Marvel Universe.
Juggernaut is a professional assassin who doesn't care who he snuffs out. The gem of Cyttorak turned Marko into an unstoppable living engine with the ability to smash through any barrier, and making him immune to pain and injury. With Juggy and Cyttorak's gem now one, Juggernaut is more powerful than ever, and may even be immortal. He also has the power to from a force-field around himself for protection. His helmet is armor that is bullet-proof offering more protection. However, the helmet limits his peripherial vision.

X-Men #12