Diffutils

Diffutils are a GNU collection of diff utilities.

Utilities

diff: diff compares two files and shows the differences, line by line.
cmp: The cmp command shows the offset and line numbers where two files differ, or cmp can show the characters that differ between the two files.
diff3: The diff3 command shows the differences between three files. Diff3 can be used when two people have made independent changes to a common original; diff3 can produce a merged file that contains both persons' changes and warnings about conflicts.
sdiff: The sdiff command can be used to list diff of two files side by side or merge two files interactively.

The easiest way to keep track of your patches to your code is to use diff. E.g.:

diff -urNP foo bar > patchset_20071228.patch