A two week's notice is a resignation letter, in which you politely advise your employer that you're leaving your job, It may be a one, two or three week's notice, it means that within one, two or three weeks you won't be in your charge anymore. I think that by doint it you sort of give a warning to your employer and depending on how much time you intend to continue working it might ensure your payment for the month as you actually finish whatever your duty was in the company.
Look what I found:A resignation letter may help you maintain a good relationship with your employer while paving the way for you to move on. Use the letter to leave your employer with a strong and positive final impression.
source: http://jobsearch.about.com/od/jobsearch ... letter.htm