Field vs. Market

A market is a segment of the population who will pay you for a good or service. There is a demand there.

A field is a location in time or space, or a segment of the population, in which your action is called for. There is a demand or implicit demand there as well.

The difference is that a market is specifically oriented toward making money, deriving self-benefit. If you're interested in profit, you're interested in markets. Whereas a field is oriented toward making use of your ability to do things. If you're interested in doing things, you're interested in fields.