On a related note, the website cheat.sh is also a great resource. Just curl it with the command you want to learn about as the endpoint.

For example, if I want to learn about grep, just open a terminal and

$ curl cheat.sh/grep

And a short and sweet description with examples will be returned.

See also:

• tldr — collection of community-maintained help pages for command-line tools
• explainshell — write down a command-line to see the help text that matches each argument
• General purpose command-line tools — examples for most common usecases
• Bash reference cheatsheet — nicely formatted and explained well
• Bash scripting cheatsheet — quick reference to getting started with Bash scripting