The By-Sa (Attribution Share Alike) license lets users remix and modify your work even for commercial reasons, provided that they credit you and license their new creations under the same license. Any new works based on yours will have the same license, so any derivatives will also allow commercial use.
By-Nc-Nd (Attribution Non-Commercial No Derivatives) is the most restrictive; it allows downloading and sharing without remixes or commercial use.
By-Nc-Sa (Attribution Non-commercial Share Alike) lets users remix work non-commercially under the condition that they give credit and license new work under the same license.
By-Nc (Attribution Non-commercial) is similar to By-Nc-Sa except the derivative works don't have to be licensed on the same terms.
By-Nd (Attribution No Derivatives) allows any kind of redistribution as long as credit is given to the creator.
By is the Attribution license which allows users to do anything they want with a work as long as credit is given to the author of the original. This is the least restrictive license.
More details on these licenses can be found at Creative Commons Licenses.
