The command C-h p (
finder-by-keyword) allows you to browse
the packages that come with Emacs.
For advice on how to find extra packages that are not part of Emacs, see Where can I get Emacs Lisp packages that don’t come with Emacs?.
Other techniques that might be useful:
Typing M-x apropos RET python RET lists all functions and variables containing the string ‘python’.
You can look through your computer’s lisp directory (see What are src/config.h, site-lisp/default.el, etc.?). The Lisp source to most packages contains a short description of what they do and how they should be used.