Pretty much what jmarkb says.
For the purposes of topography in Scotland there is no such thing as a col, or at least it occurs extremely rarely.
Bealach can mean any gap, pass, low point between hills. So whilst all cols are bealachs, not all bealachs are cols
It also has a wider meaning of a way or passage or a road. For something that is specifically a high mountain pass we also have the word lairig.
Also, can we all stop saying bee-lach