According to Nolo's Plain-English Law Dictionary, a license is "permission to do something otherwise prohibited under law."
In addition to the exceptions to copyright law, various types of licensing exist that allow for regulated use of a copyrighted work. Sometimes the copyright holder places a license with a notice on the work itself (such as with Creative Commons licenses). Other times a license can be obtained through a clearinghouse or publisher (such as with the Copyright Clearance Center), as is often done by libraries. See our tab on Pre-Licensed Works for more details on licensing.
If your use falls outside of an existing exception or license, you may need to contact the copyright holder directly to seek permission—this is like securing a very specific, personalized license. If you seek permission from the copyright holder, you will need to work out the terms of your particular use and make sure you adhere to what is agreed upon. See our tab on Seeking Permission for guidance on this process.
Licenses may or may not come with a fee, and all licenses have different stipulations as to what can be done with the work. In some cases, a license may be more restrictive than copyright law, and in others, there may still be room to make use of exceptions like fair use. If you are using a work under a license, it is important that you understand the details of that particular license.