Reggae has, in general, the following characteristics (though there are always exceptions, of course):

1. 4/4 time with, as Ray says, the drum beat on the 3rd beat of the bar rather than the 2nd and 4th
2. guitar plays 'choppy' chords rhythmically on the 2nd and 4th beats of the bar (where the drummer would play in pop music)
3. often a much more interesting and melodic bass line than in pop music, the bass often 'driving' and/or underpinning the song much more than in many other forms of music
4. a particular 'language' in the words, often due to or derived from Rastafarianism (though I think this is far less true now than it used to be, as the music has, to a large extent, moved away from its Rasta roots into pop culture)