A quick google search:
Numerical Differential Ability Scale
This differential aptitude test measures your capability to interpret numerical relationships between different figures.
Your mathematical reasoning is also tested. You can improve your performance by going through basic mathematical manuals and science hand books.
Abstract Reasoning Differential Ability Scale
Abstract Reasoning (AR) is not your verbal fluency...
This differential aptitude test measures your reasoning when you solve problems in terms of size, shape, position, quantity or other geometric figures and shapes. Logical thinking is involved.
Mechanical Reasoning Differential Ability Scale
Mechanical Reasoning (MR) measures your ability to understand mechanical principles of machines, motion and devices.
It is an important test for engineering and machine operation jobs. It involves logic with motion. However, specialized knowledge is not required.
Space Relations Differential Ability Scale
Space Relations (SP) measures your capability to analyze three dimensional figures. How do you see 2D or 3D drawings? When you are looking for some job in engineering, architecture or designing you must expect such kind of aptitude test.
These are all sorts of things you can't really study for, you have these abilities or you don't; you can always 'bone up' on your maths... The number 60 is generally the key in ATC maths...