From LQWiki

ctags, the current version of which is actually named "Exuberant ctags" is an enormously useful program that generates a tagfile from whatever source code files you throw at it (for example, all the source code files in a project you might be working on).

Then, using your favorite editor, you load the tagfile which then enables you to hop around your source code (across files and directories even) quickly tracking down function declarations, variable definitions, and so on.

This is in contrast to trying to remember which file contained which declaration/definition, and/or grepping through your source directory looking for such things. ctags saves you lots of time.

See also

• etags