No, I didn't spell parole incorrectly... the rather uncommonly used word parol is used in the same sense as one would use the word oral or verbal. This rule, effective in United States courts, states that you may not give parol evidence in court to vary or contradict the otherwise unambiguous terms of a written contract. You may, however, explain or clarify something ambiguously written, or note that there was a verbal agreement in addition to the written contract.

Say, for example, that you sign a contract with your dealer, to the effect that you agree to pay him $10 for a dimebag next Tuesday. As you're looking over the document, you happen to ask him whether there will be any seeds or stems in the sack, and he promises you that he'll go through and take them out. If it turns out that the sack he presents you with when Tuesday rolls around is chock full of seeds, you can take him to court*, and testify that there was a verbal agreement in addition to the written one, whereby he promised to de-seed the weed. Of course, you would want to back up your case by calling any witnesses who might have been present at the time, or referring to past instances where he has promised and delivered seedless products.

But say that there is a small line of text at the bottom of the written agreement, which stated that "Removal of seeds and stems is not guaranteed." You ask him about this line before you sign the document, being the careful emptor that you are, and he replies that there's no reason why he wouldn't remove them... it is part of his stash, after all. If he gives you a seedful sack, and you try to take him to court, you will not be allowed to say that such a verbal agreement took place, because that would contradict a very straight-forward statement which you agreed to in the written contract.

The moral? If someone tells you to sign a written contract whose terms are in conflict with a verbal agreement you've established, DON'T SIGN THE CONTRACT. This gives the other party a perfect opportunity to fuck you over and remain legally immune.

Many written contracts have a merger clause, stating that there are no agreements between the two parties outside the terms presented therein. These are useful for avoiding sticky situations like the one presented above, but you should take note if there is such a clause in a contract you're about to sign, and make sure that anything you want out of the deal is stated explicitly on paper. Remember, from a legal point of view, all contracts are negotiable... if handwritten changes are made on a typed contract, and there's a discrepency between the two, the terms that are written by hand are the ones that apply (unless you can prove that someone altered the contract after it was signed... that's against the law).

* this is probably a bad idea, though