To renege is to go back on a promise, to make a denial, or revoke something. It is pronounced many different ways. The first syllable may have the long or short e sound. And the (nege) syllable can be /nehg/, /nahg/, or /nig/.
A person who reneges is a reneger.
It is also used sometimes in board or card games when a player has broken the rules. In the game of euchre, you play cards by suit. If you play a card later, but should have played it earlier in the game, you reneged on the promise that you had no more of that suit.
Rescind is a verb that means to officially terminate something, like a law or contract. It may also mean to repeal or cancel. A person who rescinds is a rescinder. An uncommon noun form is rescindment.
The difference between the two is mostly that rescind is more formal and renege is looser. Therefore, rescind is found more often in written texts.