$ man apropos
  
  apropos searches a set of database files containing
  short descriptions of system commands for keywords and
  displays the result on the standard output.

It answers the question "I wonder how to do [this English word]."

For example: "I wonder how I copy a file."

  $ apropos copy

Dumps out all of the matching entries from the man pages for the word "copy". If you just wanted general commands, as opposed to library functions, and assuming your system organizes man page sections in the typical manner, you can filter it down with:

  $ apropos copy | grep "(1)"
  cp                   (1)  - copy files and directories 
  cpio                 (1)  - copy files to and from archives