I learned this a long time ago. Never listen to Mysterio. Not because he tends to go on and on (which is true)... He's got illusions, the gas, but the helmet gives his speech pattern some crazy mind-bending properties.
Quentin Beck was born in Riverside, California. Before becoming Mysterio, Beck was a special-effects wizard who worked for a major studio and had dreams to make a name for himself in the film industry. Beck began his career as a movie stuntman in Hollywood, then became interested in special effects. He became one of the most accomplished special effects designers in the industry, but due to lack of recognition, he wanted to become an actor. However, he lacked the looks and talent to be a star and the patience to be a director, and saw his career in special-effects to be a dead-end job. When a friend of his jokingly suggested that the quickest way to become famous was to take out a costumed hero, Beck realized that his expertise in illusions could make him an effective super-villain. Choosing Spider-Man, then a relatively weak and inexperienced target, Beck prepared his resources for a few months before attempting to eliminate the wall-crawler. Mysterio, his chosen identity, became one of Spider-Man's most elusive and persistent foes.
Beck studied Spider-Man, wanting to emulate the wall-crawler and eventually take his place as a costumed crimefighter. Under the guidance of the Tinkerer he became a convincing alien, but was defeated by Spider-Man.
Later, he posed as Spider-Man and created the identity of Mysterio, but again was defeated.
The Sinister Six
Unable to best the wall-crawler on his own, Mysterio sought strength in numbers. Early in his career, he and five other members of Spider-Man's rogues gallery united under the leadership of Doctor Octopus -- brilliant, respected scientist turned tentacled terror. The inaugural incarnation of the Sinister Six also included the shifty Sandman; Kraven the Hunter; the high-flying, lowdown Vulture; and Electro. The villains' master plan: Pummel the wall-crawler one by one until one of them would manage to defeat him. The outcome: Spidey 6, bad guys 0. 
He become one of Spider-Man's more frequent villains. He once took on the alias of psychiatrist Ludwig Rinehart and nearly convinced Spider-Man that he was insane, and should reveal his secret identity.
In later years, he was succumbing to cancer and decided to drive a hero insane, choosing Daredevil. After finding out Daredevil's true identity, he brought together an elaborate scheme that entailed portraying an innocent baby as the Anti-Christ,framing Foggy Nelson for murder, and tricking Matt Murdock's ex-lover Karen Page into thinking she was a carrier of H.I.V. due to her past as a porn star. After hiring Bullseye to locate Daredevil and the baby, Karen Page died while protecting Daredevil from him. Eventually, Mysterio was confronted by Daredevil, but when Daredevil refused to give the illusionist the pleasure of killing him personally, he decided he had nothing else to live for, and took his own life.
After committing suicide, Beck supposedly came back from Hell in a red Mysterio suit to wreak havoc on Peter Parker's life after Peter was unmasked. Beck took off his helmet at one point to show the giant hole in his head from where he had shot himself, showing he was still dead. It is yet to be seen how he was animated or what sort of new powers he has, or whether this was just an illusion.
More recently, Mysterio helped Norman Osborn fake Harry Osborn's death. He has seemingly come back from the dead, and is back in his original green suit.
Again a part of the Sinister Six, Mysterio helped Doctor Octopus with his plans of stealing technology. He also participated in the battle against Intelligencia, where his group stole the Zero Cannon and almost wiped out the whole rival team.
Later, Doctor Octopus, in his plan to be remembered as the man who saved Earth, offered the chance to stop the Greenhouse effect, as well as speed it up, using a special device connected with different satellites on the orbit. Disguised as Al Gore, Chameleon tried to accelerate the decision of world's leaders about the matter, but was uncovered by Spider-Man, who irrupted the scene along the Avengers. When the Chameleon was freed, he arrived in the Mediterranean on a meeting with the rest of the team. The Avengers followed them and arrived, fighting the Sinister Six, but they where all defeated by the villains, though Electro was sent into space by Thor.
Using an explosion as a distraction, Silver Sable saved Spider-Man and Black Widow. While trying to get to one of Octavius's manufacturing plants to access a satellite, they defeated Sandman; Octavius responding by asking for the help of all nations, due to his position in the matter, in capturing Spider-Man. As he was trying to defeat the heroes, Beck was convinced by Spider-Man to join them. After a fierce battle against the Avengers, who were being controlled by Octavius, Mysterio revealed Doc Ock's hideout and fled.
Killing the Spider-Men
Some time later, it was revealed that Mysterio found a way to enter another universe, and began using an android in hopes to conquer this world. Mysterio was found by Spider-Man when his portal caused a massive beam of energy; during the ensuing battle, Mysterio damaged his interdimensional device, causing that Spider-Man to end up in the other universe. Mysterio activated his remote-controlled avatar and used it to hunt and destroy the Spider-Man of his world and the current Spider-Man of the other. He managed to find the two at the Triskelion, and waged war against them.
Beck escaped the scene after battling the Spider-Men, and brought the android back to his universe. He had the temptation to return the android to the other universe and fight Spider-Man once more, as he was now trapped in that world, and Beck had managed to create a world without Spider-Man. What he didn't expect was the Spider-Man waiting for him in the other universe, who defeated and incarcerated him in the other universe.
When Galactus broke through the barrier between Earth-616 and Earth-1610, and vaporized New Jersey, he was interrogated by Iron Man, where he revealed that Reed Richards of Earth-616 had stopped Galactus before on several occasions. He was seen to be quite disoriented, indicating that traveling into an alternate universe and remaining there could cause health problems.
Beck was a master of make-up, physical stunt work, mechanical and visual special effects. He knew basic psychiatry and could employ hypnotism successfully. He also learned a great deal of applied chemistry, especially the use of hallucinogens, and electronics. Beck was a meticulous planner and organizer, and a skilled actor who kept himself in top physical form.
Mysterio (in any incarnation) has personal weaponry that include a one-way acrylic glass helmet (with 30 minute air supply) and holographic projector, and gloves and boots armed with nozzles which emit hallucinogenic gas. He developed a gas that can cancel Spider-Man's spider-sense, and when he is cloaked in his mist gases, he uses sonar to detect objects and beings nearby. He also has more advanced technology (not of his own design) at his disposal. Mysterio has also created his own version of Spider-Man's webbing, including a gun that fires webbing, but lacks the complete knowledge of the formula Spider-Man uses, since Spider-Man had also designed his artificial webbing to begin dissolving after an hour has passed.