Documentation
Programmers Manual: man: /usr/man. Use also $man -k or $apropos
info pages -hypertext browsable texts, often identical or updated versions of man pages.
/usr/doc/program-name: ASCII/ html docs installed with the named program.
Howtos : Tutorials on Linux- related topics, available on- line if installed (in /usr/doc).
W W W: for recently- released programs, authorised & third- party sites.