Semi-sweet chocolate is made of:
cocoa liquor (at least 35% in the States), sugar, cocoa butter (at least 40%) and vanilla.
It is most closely related to bittersweet chocolate, though semi-sweet contains more sugar, giving it a milder flavor. Semi-sweet chocolate is particularly popular in baking because of its subtle taste; most chocolate chip cookies are semi-sweet or milk chocolate.
Info from http://www.joyofbaking.com/SemisweetChocolate.html