A knot is used to join two ropes together or a rope to itself.  If done correctly a knot will hold shape regardless of it being fixed to something else.  A hitch is used to fix a rope to another object, such as a carabiner or pole, and relies on that object to hold.  You can see this easily by tying an eight follow-thru onto a carabiner.  Do the same with a clove hitch.  Now take the carabiner away and see what happens – it will fall apart.  Some sources classify a hitch as a class of knot but the general distinction remains the same.