Assuming from your user name you're in the UK, here's the answer for that market:

A 'Nett' fare is the industry term used for the fares that airlines distribute to travel agents for them to sell.

Whenever you buy a fare through a travel agent in the UK (that is not a published fare) it will be a nett fare.

These fares are issued in a number of different booking classes and are not a booking class in their own right.