Off beats are the ones played in between or against the rhythm, good example is " sexes on fire " intro guitar from kings of Leon, in modern rock music the accents lay on the second and fourth beat so you could call them the strong beats, however in reggae the accent lays on the 3rd beat making that the strong recognizable beat so it all depends on the kind of music to, my advise is don't worry about all of that, it will come naturally.