http://unixhelp.ed.ac.uk/
UNIX Help for Users.
This page contains 4 sections: Tasks, Commands, Concepts
and Utilities.
Each section contains sub-topics with explanations and examples. It has
a very good explanation of the
vi editor (
http://unixhelp.ed.ac.uk/vi/index.html)
with the description of the commands and how to use the vi editor.