Affiliate marketing is a bit like being a sales rep for another company.
This could mean that you do some sales pattern with a potential customer, or that you say hey Mrs. X meet Mr. Y, he has an awesome product that I think you'll like. And walk away.
Obviously you don't actually meet anybody. Instead you can get visitors, or traffic from somewhere, and send it to offers. If the visitor to the offer buys, you get a percentage of the sale. These differ from seller to seller, and offer to offer. Some may give you 100% of the initial offer, and they take the rest, others a percentage of everything that visitor buys.
From an income perspective, its good to get a percentage on recurring incomes. That is incomes that get will (and paid to you) every month.