Thc is but one of the components contained in the resin. By resin I mean the trichomes found on the bud. Hash is a concentrate of resin, made the purest possible using water extraction, but it can also be made with a screen though it is much less pure usually since small particles of plant matter get through the screen. Resin is actually mostly found on the bud, and often quite a bit on the small leaves around the bud.

Like you I used to think hash was something else, made from the seive of the plant or something, but it is simply a mass of the capitate trichomes found on the plant.