Mr. Satan became the World Martial Arts Champion before the Cell saga. When the Cell Games began he entered but soon lost. The day after however all the papers said that he was the one who destroyed Cell. Just before Majin Buu
was about to be awakened a group of actors made a show how they thought the Cell Games turned out. When all the Z-fighters except #18 leaves she makes a deal with him saying if she threw the match that he would pay her twice the prize money. After Buu was threatening the people Mr. Satan went to his house and ended up becoming friends. This is when Buu spit out the evil Buu which soon absorbed the good Buu creating Super Buu.
In GT Mr. Satan is still the champ. His daughter Videl is married to Son Gohan. He also has a grand-daughter named Pan. He is still considered the strongest man in the world, but that's for the humans that don't know the Z fighters.
Mr. Satan doesn't have any real powers, he has a set of gadgets he uses such as bombs disguised as game consoles. He is rumored to use a jet pack for flight capabilities but he is hardly seen using it. Though Mr. Satan is not as strong as any of the Z Fighters he is strong for a normal man but his daughter Videl is much stronger than he is since she trains more than he does but Mr. Satan is strong as he demonstrated his strength by karate chopping 5 layers of wood, he is an exceptional hand to hand combatant but his skill is below almost every fighter in the Dragonball Z universe.
Mr. Satan is called "Hercule" in the english versions, most probably because of the reference to, well, Satan. In a recent guide Akira Toriyama revealed Mr. Satan's real name. This is "Mark" which is pronounced in Japanese as "Maaku" and when you scramble these letters, you'll get "Akuma" meaning "devil" in Japanese, maintaining the pun. Mr. Satan is only his stage name.