A good place to start is the Creative Commons website (see links). For text, you can look at the Gutenberg project.
Public domain materials have no limitations.
A public domain application is one for which protection has expired (very unlikely) or which the creator has donated to the public domain. In computer applications, it is far more common to find Creative Commons- or GNU-licensed materials.
Yes; all materials prior to 1923 are in the public domain.
Materials can enter the public domain when copyright term expires, protection is not renewed (renewals are not available for new works), the creator donates the work to the public domain, or the materials are a work of the US Government.
Yes; materials in the public domain do not require any licenses.
Use only materials for which you are the rightsholder, materials in the public domain, or materials for which you have a license.
Yes; materials in the public domain have no protection under the copyright law.
The public domain enriches the culture by giving creators a free toolbox of older materials to start from.
Yes; in the US, materials published prior to 1923 are in the public domain.
No: they are free for public use.
Yes; materials published prior to 1923 are in the public domain. Dulcy premiered in 1921.
Materials protected by copyright may only be used by their creators or rightsholders; anyone else wishing to use them must get permission. Materials in the public domain are available for free use for everyone.