Mike has it spot on! Weather helm is caused from the balance of the sails in relation to the rudders, hulls, centreboards OR more correctly, the relationship between the centre of lateral resistance of the boat to the centre of effort of the sails. If the CE is too far forward (relative to the CLR) then lee helm is the result, too far back and weather helm occurs.
A crew can also effect the helm (as most sailors are fully aware), when the crew is forward the bow goes down, the stern comes up and the CLR "moves" forward, boat point higher – weather helm -, similarly when moving back the reverse and weather helm reduces. IE lee helm -
Rake the mast back excessively - weather helm - too far forward - lee helm -. Let the main off to windward and pull the jib on - lee helm (or the more noticeable result, - the boat sails from a beat onto a reach and all the way down to a run), pull the main on, let the jib off – the boat sails itself “up” even to the point that some boats will actually tack without any helming by the crew. IE weather helm.