Most optional features in Emacs are grouped into packages. Emacs contains several hundred built-in packages, and more can be installed over the network (see Packages).
To make it easier to find packages related to a topic, most packages
are associated with one or more keywords based on what they do.
Type C-h p (
finder-by-keyword) to bring up a list of
package keywords, together with a description of what the keywords
mean. To view a list of packages for a given keyword, type <RET>
on that line; this displays the list of packages in a Package Menu
buffer (see Package Menu).
C-h P (
describe-package) prompts for the name of a
package (see Packages), and displays a help buffer describing the
attributes of the package and the features that it implements. The
buffer lists the keywords that relate to the package in the form of
buttons. Click on a button with mouse-1 or mouse-2 to see
the list of other packages related to that keyword.