Che Vive is the only one who is correct. Ska is just one step in the evolution of reggae. First in Jamaica there was Mento and other Caribbean styles, which developed into Ska during the late 50's early 60's with the influence of American R&B music. Some examples are Price Buster, Very early Wailers, Desmond Dekker, and the Skatalites. By the late 60's, the music had slowed down and was called Rocksteady but was still largely influenced by R&B. By the early 70's it had slowed down again, the lyrics were more political, social, or culture based, and the new sound came to be known as reggae.
2nd wave ska (2tone era) started in the late 70's, with bands such as The Specials and Madness. It was a revival of the Ska sound. The second revival was the 3rd wave, which started in the 80's with bands like Might Mighty Bosstones and lasted through the mid to late 90's with No Doubt, Reel Big Fish, The Slackers, etc. There was also a lot of ska punk and ska-core throughout this era, for example Operation Ivy, Voodoo Glow Skulls, and Suicide Machines.
Most young people are only familiar with the 3rd wave, and sometimes also the 2nd.
To answer your question, if you listen to any ska band mentioned above, then listen to a reggae artist (try Bob Marley, Buju Banton, Black Uhuru, Steel pulse, etc) you will be able to clearly hear the difference.