Hi Ravindra,
To summarize it all up, CATT which stands for Computer Aided Test Tools are scripts that contains the master data for the systems. There's a vast kinds of CATT scripts that comply to certain sys/module (eg, zt_sap01,zt_scm601..etc.). There are two types of it namely catt and e-catt. It contains all the tables and data needed for the course.
Hope this helps!
Regards,
Shilla Gold