Jeans are trousers made of denim, usually dyed blue (see Wikipedia:Denim). Until the advent of designer jeans in the early 1970s, Levi-Strauss dominated the market and the term Levi or Levis is still used to refer to a manner of dress, particularly blue denim trousers and jackets.
The original and classic jeans were reinforced with copper rivets, and Levi-Strauss had a major market with a very popular line with metal fasteners in the crotch instead of a zip. They still produce their "501" model, which remain a de regueur fashion statement: mostly worn by men, they have the advantage that the flies are buttoned shut, so the penis can be pulled out without danger of chafing by a zip, and also means they are more comfortable when not wearing underwear.
In the Leather scene, Leather-Levi, often abbreviated LL, indicated a venue where butch behaviour and dress were the rule (as opposed to places like piano bars). LL did not necessarily indicate a serious S&M venue.
Leggings designed to look like very tight jeans are called jeggings.