Depending on what you intend to do with a copyrightable item, you may need permission from the copyright holder, or it may fall under Fair Use.
A copyright owner has the right to control a lot of uses of their works, such as making copies (of all or part of a work), distributing copies, showing or performing a work in public, and making new works based on existing ones. While there are limitations and exceptions to copyright owners' rights, in some circumstances, many types of uses fall under exception or exemption to copyright law. Other times, you may be able to use something because your purpose fits within Fair Use, a flexible part of the law that enables many different types of uses depending on the circumstances.
Links to resources that provide an overview on copyright laws and policies.