Mootools Core and Mootools More are the core items in the Mootools frame work. At the least you need Mootools Core. If you wish to expand out and add on some functions like a tip window then you could add on the More library which is the collection of official extensions to the Core.
Clientcide is a library made by Arron newton, who is one of the Devs of Mootools. It contains a large collection of useful classes. It is not exactly an Official addon but you will be hard pressed to not get support for it on here :) As it is rather handy from time to time. The Forge is a section of the Mootools site where 3rd party devs like us can upload add on Classes/plugins and libraries for people to use. These add-ons are built using the Mootools Core/More , the Clientcide libraries , or other classes from the Forge. It makes for a nice clearing house of work people have done and wish to share as opposed to remembering 100's of links to the things you want.
