Shell is more than just a command processor

Don't be discouraged by the open-ended nature of this topic.
"Mastering" shell usage is indeed a large and long undertaking.. 
..but..
..there is a lot to gain from even small investments of study and effort.

The wikipedia article http://en.wikipedia.org/wiki/Shell_(computing)offers this family-classification .. probably subject to alternative viewpoints and basis of classification

• sh .. ash dash bash ksh fish
• csh .. tcsh
• es
• rc
• schs
• zsh
• sash

Loose thoughts on ways to do more than execute a single command

• Filesystem navigation (pushd, popd, dirs)
• pipelines and redirection ( * | && || < > >> &> exec )
• jobs: job-control (jobs), background (&, ^Z, bg), foreground (fg), signals
• aliases, functions, environment variables
• Scripting

• cmd vs (cmd) vs { cmd; }
• sh -c 'cmd',
• sh scriptfile vs source scriptfile

• Misc

• shebang, subshells, environments, return codes
• globbing, word-splitting, expansion(s) and substitution(s)
• semicolon ';' command-separator
• command-terminator/separator (terminology?) &, &&, ||
• quoting and escaping
• shopt
• debugging

References

• http://wooledge.org/mywiki/BashPitfalls .. avoiding problems
• http://www.linux.com/articles/34658 .. a few words about portability
• http://wooledge.org/mywiki/BashFAQ .. a pretty good FAQ page
• ftp://ftp.cwru.edu/pub/bash/FAQ .. an older FAQ (I think the author may have been a major contributor)
• http://www.tldp.org/LDP/abs/html/ .. Advanced Bash-Scripting Guide .. loaded with examples