What is Copyright?
Copyright is the legal protection (specific rights) authors/creators can benefit from for their works of creation that are eligible for protection.
For consumers of others' created content, Copyright is a form of Intellectual Property that determines how others' content can be used. For example, content cannot be reproduced or used for commercial purposes without clear permission. Copyright enables innovation in artistic and scientific fields. The rights-holder can gain financially for her/his work because consumers cannot use their work without seeking permission. It is the expression of an idea in tangible form - paper, film, or silicon chip - that can be copyrighted.
Copyright laws may be a little different in different countries. In USA, domestic works need to be registered so that rightsholders can sue for infringement, when required.
Copyright of literary, musical, dramatic, and artistic works protection has a lifetime: typically, it lasts for the entire lifetime of creators plus 70 years. It's popularly known as the 'life plus 70' rule.
What is Public Domain?
Public Domain refers to all creative works that are not protected by Copyright.
These works can be used and re-used and transformed according to the wishes of content users. There is no need for seeking permission.
No one owns works in Public Domain.
For example, creative works by authors whose term of copyright lies beyond the 'life plus 70' rule belongs to Public Domain. Shakespeare's works are in Public Domain.
All governmental websites with information for public consumption are in Public Domain.
What is Fair Use?
Fair Use is that doctrine in US Copyright Law that allows content users to use a certain percentage of copyrighted work for specific purposes such as, teaching, criticism, news reporting, and research, without seeking rights holder's permission.
There are Four factors that courts and judges examine to determine cases of fair Use