qmake is part of the Qt library's supporting tools. It is used to generate platform/environment specific build instructions for a project from a project description (*.pro) that is (mostly) platform independent. The build instructions are typically output as a Makefile for a make utility.

CMake is a third-party tool that provides a similar cross-platform build capability. Given a project description in CMakeLists.txt CMake will generate Makefiles (generally) matching the specified target tool set that can be used to build the project. KDE is a user of CMake.

I think Qt beginners should start with qmake.