KDiff3 is a KPart. Currently it implements the KParts::ReadOnlyPart-interface.
It's main use is as difference-viewer in KDevelop. KDevelop always starts the internal difference viewer first. To invoke KDiff3 press the right mouse button on the difference viewer window and select "Show in KDiff3Part" from the contextmenu.
KDiff3 normally requires two complete files as input. When used as part KDiff3 will assume that the inputfile is a patch-file in the unified format. KDiff3 then retrieves the original filenames from the patch-file. At least one of the two files must be available. KDiff3 will then invoke patch to recreate the second file.
In Konqueror you can select a patch-file and select "Preview in"-"KDiff3Part" from the contextmenu. Be aware that this won't work if none of the original files are available, and it is not reliable if the original file(s) have changed since the patch-file was generated.
When run as a part KDiff3 only provides the a two-file-diff, a very small toolbar and menu. Merging or directory-comparison are not supported then.