There are a few exceptions to this rule. For example, if a work is in the public domain, it is free for anyone to copy. Works in the public domain include works whose copyright has expired, works that were never copyrighted, and works that have been dedicated to the public domain by their authors.
There are also a few fair use exceptions to copyright law that allow for limited copying of copyrighted works without permission. For example, it is generally considered fair use to copy a small portion of a copyrighted work for the purposes of criticism, comment, news reporting, teaching, or research.
However, it is important to note that the fair use exceptions are narrowly construed and copying a copyrighted work without permission is always a risky proposition. If you are unsure whether copying a particular work is legal, you should consult with a copyright attorney.