Ska originated in Jamaica and eventually made its way to the U.S. somewhere in the sixties. This first sound of ska was very mellow and rocksteady sounding. In the 70's and 80's ska took a new form with bands like The Specials. It was slightly more upbeat still reminiscent of reggae. It was popular in the U.K. though. Somewhere in the late 80's someone decided to mix punk and ska and thus we got the modern ska sound we know today.
Ska is classified by the use of only the upstroke on the guitar, giving it a "happy" sound. It was a style of music that was made for dancing. If there are any real ska fans left on this planet I'm sure they love to go to shows and skank their hearts out.