Who is Ant-Man? There have actually been three different men to wear the Ant Helmet. The first and most famous is Hank Pym, a brilliant scientist that discovers a form of subatomic particles that he dubs "Pym particles." These particles allow him to shrink down to microscopic levels. After exploring his surroundings as a pint-sized adventurer, he develops a hi-tech helmet that allows him to telepathically control all ants. Eventually, Pym realizes that his particles work the other way too, and he assumes a second superhero identity: Giant-Man. So in case anyone was still not taking him seriously, he's now got the ability to squash them.
The second Ant-Man is Scott Lang, a single father who works as an electronics expert under Tony Stark. When his daughter gets seriously ill -- and the only doctor who can save her is kidnapped -- Lang has no choice but to steal Pym's Ant-Man tech and rescue the doctor himself. Pym is so impressed by by Lang's skills that instead of having him arrested, he allows Lang to continue being Ant-Man.
The third Ant-Man, Eric O'Grady, is known as "the Irredeemable Ant-Man"; he's a low-level S.H.I.E.L.D. employee that steals an Ant-Man suit and proceeds to do nothing with it beside leering on women in the shower and stopping crime in exchange for cash and girls' numbers. The Avengers eventually catch on to his act and begrudgingly try to whip him into shape.
In addition to Ant-Man, we also have to mention Janet Van Dyne a.k.a. The Wasp. Janet starts out as a flighty fashionista socialite whose father works with Pym. When her father is killed by a science project-gone-evil, Pym and Van Dyne team-up to defeat the threat. After being subjected to Pym particles, she can also grow to microscopic size, and develop insect wings and a powerful "wasp sting" energy blast. Pym and Van Dyne start out as partners with unrequited feelings of romance, but they eventually become a couple. Then they get married. Then they get divorced (saving the Earth is quite taxing on a marriage).
All four heroes have been Avengers at some point or another.
Who is the Villian? Ant-Man doesn't really have an amazing rogues gallery. Back in the day, he battled with Egghead, a mad scientist that... had an egg-shaped head (stop laughing).
Pym's greatest foe, however, arises during his stint with the Avengers. Ultron is a robot creature that was developed by the scientist and infused with his own brain patterns. However, Ultron eventually gains -- like all robots eventually do -- intelligence and rebels. He considers Pym to be his "father" and becomes attracted to the Wasp. Ultron grows to be a serious Avengers bad-guy and Pym, blaming himself for the villain's path of destruction, is always there to stop him.
What Is the Movie About? With Edgar Wright ("Shaun of the Dead," "Scott Pilgrim") onboard, expect a more comedic take on the character -- one that embraces the silliness involved with an ant-sized superhero. And while plot details are few and far between, it looks like the movie will involve not just Pym, but also Lang or O'Grady stealing his Ant-Man tech.
What Should I Read? Ant-Man always works best as a team player, and as such, his best stories involve team-ups with other heroes. The books that best match the odd-couple comedy-action feel of Edgar Wright's upcoming movie will probably be either Robert Kirkman's "The Irredeemable Ant-Man" or Tim Seeley's "Ant-Man & Wasp: Small World."
For his most heroic adventure ever, read "Avengers: Ultron Unlimited"; in this story, the robot nemesis gets a dangerous upgrade that turns him into a global threat.