The reef (or square) knot: This is the most commonly used knot. Not used for tying somebody directly, but for fastening other bindings.
How to make the reef (or square) knot:
Twist one rope over the other and then twist back, forming two interlaced loops;
This, as most knots, works because the rope passes twice by the same loop in opposite directions, and both segments are pressed one against the other when the loop tightens;
So, note the rope that leaves the lower twist on the upper side enters the upper twist also over the other rope;
Tighten by pulling by the loose ends (as already stated before);

Be careful not to finish just with two twisted ropes (by entering the upper twist under the other rope), not a knot, which is sometimes called “granny’s knot”.

I will post some pictures of those knots so it will be easier to understand how to make them!