The User Manual is the place to deepen your knowledge of the timeLines interface, and several of the major Modules. You should have taken the Tour and worked through the Tutorial before tackling the User Manual.
The Administration Manual is an introduction to over 50 administration pages that let you: configure the timeLines interface; control privileges for all the member/users; add, edit, and delete Items and Item Groups to the various Modules; etc.
The Installation Manuals are for Consultants, Developers, and sophisticated organizations who are installing their own timeLines servers.