A Logisim library holds a set of tools that allow you to interact with a circuit via clicking and dragging the mouse in the canvas area. Most often, a tool is intended for adding components of a particular type into a circuit; but some of the most important tools, such as the Poke Tool and the Select Tool, allow you to interact with components in other ways.
All of the tools included in Logisim's built-in libraries are documented in this reference material.