Vim has a built in way to look up help for keywords -- the K command. Moving the cursor over a word then typing K will attempt to load a man page by running the man command in the terminal.

Many programming languages have manual systems that work in a similar way -- Ruby's ri and Go's godoc spring to mind -- Vim can support these languages by setting a custom command with keywordprg.

For example, the file that integrates Ruby's ri command (ftplugin/ruby.vim) uses setlocal to make ri the default keyword program:

if has("gui_running") && !has("gui_win32")
  setlocal keywordprg=ri\ -T
else
  setlocal keywordprg=ri
endif

External manual pages are opened outside of Vim, obscuring the view of the current file. Vim's documentation suggests a way to make it open in a new window instead (under :help find-manpage). By loading the man.vim file like this: :runtime! ftplugin/man.vim, typing :Man will open manual pages in a new window.