Whenever you're utilizing a library of any sorts for your project, it's important to know what you can and cannot use it for. SFML is licensed under the zlib/libpng license, which is far from being restrictive. It allows anyone to use SFML for any purposes, even commercial applications, as well as alter and re-distribute it, given that the credit for writing the original software is left unchanged and the product is marked as an altered source. Giving credit for using the original software isn't required, but it would be appreciated. For more information, visit: http://opensource.org/licenses/Zlib.