This computer chip processes all commands and moves all of the information around. This is why Windows software does not run on Macintosh machines. If there is particular software you want to run, check that the CPU you buy is compatible. Differences in CPUs will be the largest factor in the cost of a machine. If you want a large monitor, make sure that the computer you have selected can run that monitor.
Some larger monitors come bundled with an accelerator card.
Some provide built-in track balls, some provide more function key options, some are designed to be more ergonomically sound. There is no technical reason to buy one type or another, it is purely a matter of personal preference. There are track balls, mice, finger pads, and graphics tablets each of which control the on-screen position of the cursor. This is the part of the machine that is used by programs and documents you are working on. Also known as main memory, RAM contents are lost when the computer is turned off.