The TEACH Act, passed in 2002, updates Copyright Law to include distance education classes. Some of the criteria that allows educators to use the TEACH Act include being an accredited institution, limited to a specific number of students, must not be typical items used such as textbooks, and the institution must have a public copyright policy.
Public domain refers to works that do not apply to copyright law. For some works, copyright does expire. For a complete breakdown on what is public domain and what isn't, see the link below.
