Tk 8.0 is a major release with significant new features such as native look and feel on Macintoshes and PCs, a new font mechanism, application embedding, and proper support for Safe-Tcl. See below for details. There should be no backward incompatibilities in Tk 8.0 that affect scripts. The jump was made in order to synchronize the Tcl and Tk version numbers. It is a porting guide that summarizes the new features and discusses how to deal with the changes in Tk 4.0 that are not backwards compatible.
Binary releases are much easier to install than source releases.
Look in the FTP directory from which you retrieved this distribution to see if it has been updated with patches. Patch releases fix bugs without changing any features, so you should normally use the latest patch release for the version of Tk that you want. Patch releases are available in two forms. A file like tk8.0p1.tar.Z is a complete release for patch level 1 of Tk version 8.0. If there is a file with a higher patch level than this release, just fetch the file with the highest patch level and use it.