When you quit your job, you 'resign'. A letter of resignation would be the letter given to your employer stating that you intend to leave your current position on a specific date, depending on the amount of notice required.

NOTE: A letter of resignation is a document with legal status.

Your resignation means you have formally resigned, and that any entitlements you had as a result of that employment cease as of the date of resignation.

It also means that your responsibilities for your job cease at the date of resignation.