Copyright protection exists in original works of authorship fixed in any tangible medium of expression. The two key phrases here are "original works" and "fixed". The creativity level required for originality is very low - a "spark" - but is required, no matter how much effort was required to create the work.
Works of authorship include the following categories:
When the United States signed on to the Berne Convention in 1988, the previous requirements of a copyright notice and formal registration with the Copyright Office ended. Copyright protection is automatically extended to new works upon creation. Registration is required for filing an infrigement lawsuit.
According to 17 U.S. Code § 102, copyright protection does not extend to any "idea, procedure, process, system, method of operation, concept, principle, or discovery, regardless of the form in which it is described, explained, illustrated, or embodied in such work." It also does not protect lists, titles, short slogans or phrases, symbols, and the like. While these items may not qualify for copyright protection, they may be covered through other intellectual property protections such as trademarks, patents, or trade-secrets.
Works of the Federal Government are also not eligible for copyright protection and are a part of the public domain. These works are specifically defined —”a ‘work of the United States Government’ is a work prepared by an officer or employee of the United States Government as part of that person’s official duties” (17 U.S. Code § 101)—so we cannot simply assume anything posted or published by the federal government is free of copyright.
For works created and published in the United States after 1989, the copyright term is the life of the author plus 70 years.
Cornell Copyright Information Center provides a detailed chart to identify copyright terms in the United States.