United States (Government) Produced Materials
The Copyright Act provides that federal governmental materials are not protected by copyright laws. In other words, such works, by definition, are part of the public domain.
Specifically, Section 105 of the Act provides: "Copyright protection . . . is not available for any work of the United States Government . . .."
While this seems fairly simple, it can be a bit more difficult to apply in practice. For instance, a work of a contractor of the US Government is not a "work of the US Government," so it would be protected by copyright. Additionally, even if copyright laws do not operate to protect a US Governmental work, other protections (such as trademark and the like) may apply.