Glide doesn't usually mean "across a surface", but 'in the air' (think of a 'glider'). If you say that something "glides across the floor", it's often a metaphor - it means it looks like it's floating in the air, e.g. if it's moving very gracefully or smoothly. So 'slide' is the normal word for "move across a surface"; 'glide' could be use to emphasise how smooth the action is.

To glide is to to move smoothly and continuously along, as if without effort or resistance. To slide is to to move along in continuous contact with a smooth or slippery surface. They mean almost the same thing. In common usage, I would say that gliding depends less upon the surface an object is moving upon, under, or through, and in order to slide, an object would require the surface to be slippery, or slick.