tkdiff is a graphical diff viewer based on the Tk framework. It is capable of inter-operating with source-control systems like CVS and Subversion to show the differences between the local copy and the repository version. Such a line-by-line comparison is often considered to be good software engineering practice before committing code changes.
Tkdiff distinguishes itself from many alternatives by highlighting specific differences within a line shared by both files, rather than simply indicating that the whole line differs.
tkdiff <file1> <file2>— to compare the two files
tkdiff <file>— to compare the local version of the given file to the most recent version in the CVS/Subversion repository
tkdiff can also compare two older revisions of a file, etc.