There are lots of companies like Commision Junction that let you sell other people's products for a commission. How do these programs work, technically?

Ie, are you simply given a link to the product you want to sell with an embedded ID in the URL? Does it involve cookies somehow? Do you take the order yourself and forward the information to the actual supplier of the goods/services? And how do you know if the program or the 'affliate network' isn't ripping you off by not creditting you for a sale?