A comic book series owned and published by Marvel Comics. It was created by writer Kurt Busiek and artist Mark Bagley. It's one of the great success stories on how to really, really surprise an audience.
When "Thunderbolts" made its debut in April 1997, Marvel had just finished up their gigantic "Onslaught" crossover, which ended with the Avengers, the Fantastic Four, and many other heroes vanished and seemingly destroyed. Busiek and Bagley promoted their new book as the new Avengers -- the heroes who would nobly step up to fill the superhero void -- they were a brand new team of brand new heroes working to uphold the ideals of justice and heroism embodied by their fallen comrades.
The Thunderbolts included the leader Citizen V (that's "V" the letter, not "V" the Roman numeral), a swordsman and acrobat who was the grandson of one of the superheroes of World War II; Atlas, who could grow to immense size; Meteorite, who had the powers of flight, superstrength, and energy blasts; Songbird, who could form solid objects from sound waves; Mach-1, whose battlesuit gave him the flight skills and firepower of a fighter jet; and Techno, a technological genius who could build any device he needed with his special tech-pac. After battling the Hulk and the Wrecking Crew, the Thunderbolts ended their first issue by sitting together and relaxing in the privacy of their new headquarters...
...and revealing that it was all a lie. Citizen V was really Baron Zemo; Atlas was really Goliath; Meteorite was really Moonstone; Songbird was really Screaming Mimi; Mach-1 was really the Beetle; and Techno was really the Fixer. The Thunderbolts weren't really superheroes at all -- they were the Masters of Evil! The villains' plot was simple, but predictable, especially when dealing with a megalomaniac like Zemo: win the world's trust and love, and when no one expected it, take out all opposition and conquer the world!
Of course, a scheme like that can never succeed in a comic book universe, and Zemo's plots were eventually revealed to the world. However, many of the members of the team had started to enjoy being heroes, so instead of returning to a supervillain lifestyle, Atlas, Meteorite, Songbird, and Mach-1 continued to work the superhero gig, even if they had to go on the run for a while. They also attracted new members, including former Avengers member Hawkeye; Jolt, who was given energy-based powers by evil geneticist Arnim Zola; and Charcoal, who can turn into coal or diamond and generate intense heat.
After the Big Secret was officially revealed to the rest of the Marvel Universe, a lot of the buzz about the series faded away. There was no way it could match the slam-bang shocker that kicked off that first issue.
The series gets relaunched every few years -- a new team of supervillains and anti-heroes working on the side of justice and heroism, either because that's what they really want to do, or because the government is paying them to act like superheroes.