monitors

what is the difference between mainfields and nearfields?

The mains are big and far.
The near are small(er) and close.
Mid-fields are between the two.

which are better?

Aint a question of better. A mix engineer needs at least two monitoring systems so he/she can judge how well a mix translates between systems. For example. A club-banger may sound good turned up loud on the large-format mains, but may be too bass-heavy for the small format near-fields. The near fields represent more of a real-world listening platform, so most of the mixing is done on them. (also pevents hearing loss) The engineer will switch to the mains every once in a while to listen loud and to impress the client.

Depends on the size of your studio, also. Mainfield monitors won't produce an accurate reference when you're to close. So if you got a small studio, you really can't get far enough away from them.