Libraries contain symbols and imported files.

The library panel can be opened under:
Window> Library

Items can be renamed by clicking on the name.

It is useful to use folders to organize a library. To make folders, use the folder icon at the bottom of the panel. Then move things into the folder by dragging.

To delete items from the library (but be careful, they leave your file forever), select the item and hit the trash can icon on the bottom of the panel.

There is an additional options area for the library. Note the turn down arrow on the top right hand side. Click it and you will find the following:

Update Use Count: tells you how many times something is used in your movie in the library under "use count" next to the item name.

Select Unused Items: Selects items you are not using in the library which can be deleted to reduce file size. Sometimes you need to do it a few times, as symbols can be nested. (always save the file as a new name to see the file size reduction)

Properties for the selected item in the library

Linkage information that can be used with actionscript if you need it.