File comparison, synchronization, and merge utilities, from the humble Unix "cmp" and "diff" to more visual and interactive utilities. These show how two files, especially text files, are different, and often what needs to be changed to make them identical - which lines need to be inserted, deleted, changed, or moved. This helps to join two versions of a document, or to see what changed between versions.
Windows file comparison tools are generally graphical, and often compare themselves to WinDiff, a utility that comes with Visual Studio, the Microsoft programmers' tool suite for various languages.