For the full litany of the man's achievements you should of course go read his Wikipedia article, but the short story is that Stan toiled as a scribe in romance, war, and cowboy comics in the 1940s and 1950s until the late legendary DC Comics editor Julie Schwartz (who like Lee, and pretty much everyone who invented comics as we know them, was a fellow New Yorker) rescued the superhero genre from obscurity in the late 1950s. Dissatisfied with the comics he had been doing and preparing to flee the industry altogether, Stan took one last stab at the medium now that the heroes he loved were coming back in vogue, and in 1961 he wrote Fantastic Four #1. His transformative brainwave was that he wrote comics about people who were people, not flawless deities. Comics poobah Alan Moore summarizes nicely: "[DC Comics had] one dimensional characters whose only characteristic was they dressed up in costumes and did good. Whereas Stan Lee had this huge breakthrough of two-dimensional characters. So, they dress up in costumes and do good, but they've got a bad heart. Or a bad leg."
The rest, as they say, is history. Happy birthday, Stan! And thanks for making my childhood awesome.
[The Sex Pistols-inspired Stan Lee t-shirt shown here is still available from Secret Headquarters. And, of course, Shakespeare & Co. carries a wide variety of Marvel Comics publications on its shelves.]