All of a program's files are stuffed into a compressed virtual file system for easy distribution. For execution, the VFS is mounted behind the scenes, and the executable is run.
Very similar to a starpack.
Not limited to Tcl/Tk -- it supports wrapping any binary, but very limited in terms of OS support, as the resulting packages can only run on a narrow range of Linux/x86 distributions.
Note that the klik homepage refuses to show when you use Internet Explorer (or any other browser that spoofs IE).