This is the most common question which is being asked when you begin to prepare for the CAT.CAT is an exam with no predefined curriculum, but the trends of the past advices that CAT test in general aptitude including verbal ability, ability to Quantitative (math), logical resonance (LR)and tests of skill of interpretation (DI) of data.
Much like the CAT, there are other tests for admission-MAT, XAT, CMAT, and SNAP. There are some B schools (such as IFMA and IRMA), which leads their own examination. And there are state levels for admission to the State universities in their respective states.
Download CAT syllabus 2014-2015 :