Libraries contain page elements such as images, text, and other objects that you want to reuse or update frequently throughout your Web site. These elements are called library items.
When you place a library item in a document, Dreamweaver inserts a copy of the HTML source code into the file and creates a reference to the original, external item. The reference to the external library item makes it possible to update the content on an entire site all at once by changing the library item and then using the update commands in the Modify > Library submenu.
Dreamweaver stores library items in a Library folder within the local root folder for each site. You can define a different library for each site. (For more information about the local root folder for each site, see Creating a local site.)