Three different types: "Traditional", "2-Tone" and "3rd Wave". Traditional came about in the late 1950s as new dance music straight out of Jamaica. 2-Tone is named such because whites and blacks came together to form ska bands to spread unity in the 1970s. 3rd Wave is when ska hit America in the late 1980s, and added a bit of punk flair.
Also a very flexible style of music, incorporateable into other styles, which is why it's still around today and has such a large and diverse fan base.