U.S.C. stands for United States Code. You can search the code here.
The basic idea is that the resolutions passed by Congress can be found in the U.S. Statutes at Large. However, it is common practice to have potentially unexpected things in the same resolution (e.g. maybe a Defense Appropriation needed an extra vote or two to pass, and that required adding something to the resolution about a different area of law). In is the way, the Statutes at Large aren't always arranged in an intuitive way (they're arranged chronologically). The U.S. Code corrects this problem by implementing the organization scheme to which other answers refer.