Get a perfect finish everytime.
As a general rule, flat paint is used for ceilings, low sheen or matt used for walls and gloss is reserved for woodwork.
The condition of your wall and sheen level of the paint you select will determine which brushes and rollers you'll need to complete your job.
Rollers come in a wide range of nap lengths. The nap length is the length of the fibre that's attached to it. A rough wall needs a large nap length as it holds more paint.
There are two different sorts of brushes, sythetic fibre and natural fiibre brushes. The main difference between these is that natural fibres or bristles are hollow, so they hold more paint.