Men's leather belts were first invented during the Bronze Age. Women occasionally wore belts, but mainly it was the men. Between then and now, belts waxed and waned in popularity, depending on the other garb worn. For the most part, belts have been for men, with women occasionally donning them as well.
During the 1800s, men did not wear belts unless they were in the military, generally sticking to suspenders instead. The cut of slacks at the time made belts uncomfortable, so the average citizen did not wear them. That changed during WWI, when men got used to the belts worn with their uniforms, and brought that fashion back home with them, ousting suspenders for the top spot in keeping one's pants up.
During the first half of the century, Art Nuveau made a splash with specially-designed belt buckles, which today fetch a healthy sum. During the middle of the 20th century, Hollywood wanted to give cowboys a distinctive look, so they put large belt buckles on their actors, despite the fact that genuine cowboys wore primarily suspenders. This trend has bled into modern culture, with large belt buckles a piece of Western wear. Notable wearers of the large belt buckle include George W. Bush and Governor Schwarzenegger. Also, the prize given after a rodeo event is often an oversized belt-buckle as well.
Belts have generally been for function, especially men's leather belts, but women have been using belts lately for both form and function, using belts to emphasize their feminine shape. In the world wars, the Axis soldiers used belts in a similar fashion, to show off the size of their chests, as a way to decrease the morale of the enemy.
Modern belts have evolved to suit almost any fashion, from cheap webbed or cloth belts with square buckles, to the highest levels of upper class who can get diamond and gold encrusted leather belts with precious stones and custom designs for tens of thousands of dollars. Designers have created subtle belts with designs sewn in with stitching, some have printed their designs on the outside of belts decorated with metal studs, and some keep a simple strip of leather and let the buckle do all the work.
Belt buckles have also changed. In the Bronze Age, the buckle had a shield with designs of warriors or animals fighting, which later generations also did with their belt buckles. Modern buckles share a similar vision with the patriotic emblems seen on the large "cowboy buckles" with things like flags and eagles. The current box style buckle has been a more recent invention, with a piece inside of it that holds the belt in place using friction instead of a pin, which has been the traditional method for keeping a belt in place since inception.
Whatever style of belt you wear, think about the fact that you're sharing a style that millions of people spanning from today to the Bronze Ages have enjoyed.