Use library items for content that appears on many pages in your site and for content that must be updated frequently.
Placing a library item in a document inserts a copy of the HTML 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 menu.
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.)