PLR articles, or private label rights articles, are a good way to add useful, interesting, search engine friendly content to your website. Unlike resell rights, where you have the right to sell a product, but not change it in any way, private label rights afford you the right to change the product in any way you like and claim it for your own.
But there are 2 types of PLR. First is 'Restricted PLR' and the other one is 'Unresticted PLR'.
For Restricted PLR, there is some rules that you must follow. For example, you're not allowed to pass the source code to anybody else. Only you have the rights to the source code and not your reseller.
Unrestricted PLR is the one that people are looking for as you have 100% full rights to the products.
Some good PLR sites: