http://vim.wikia.com/wiki/Browsing_programs_with_tags
When editing programs, there is often a need to jump to another location, for example, to see how a function is defined. To help, Vim uses a tags file that lists each word you are likely to want, and their locations (file path and line number). Each wanted word is known as a "tag", for example, each function name or global variable may be a tag.
The tags file has to be created by a utility, and has to be updated after significant editing has occurred. This tip provides an overview of creating and using tags files (for the details, see :help tagsrch.txt).
to be continuse