external programs when programming using vim

ctags[1]:Ctags generates an index (or tag) file of language objects found in source files that allows these items to be quickly and easily located by a text editor or other utility. A tag signifies a language object for which an index entry is available (or, alternatively, the index entry created for that object).

cscope[2]:Cscope is a developer's tool for browsing source code.
* Allows searching code for:
* all references to a symbol
* global definitions
* functions called by a function
* functions calling a function
* text string
* regular expression pattern
* a file
* files including a file
* Curses based (text screen)
* An information database is generated for faster searches and later reference
* The fuzzy parser supports C, but is flexible enough to be useful for C++ and Java, and for use as a generalized 'grep database' (use it to browse large text documents!)
* Has a command line mode for inclusion in scripts or as a backend to a GUI/frontend
* Runs on all flavors of Unix, plus most monopoly-controlled operating systems.

[1] http://ctags.sourceforge.net/whatis.html
[2] http://cscope.sourceforge.net/