Affiliate Marketing:
An agreement between two websites in which one site (the affiliate) drives traffic to another site by means of advertising or content. In return, the affiliate receives a percentage of sales generated.
Affiliate Conversion Rate
The number of visitors that click through your affiliate link and buy compared to the number of visitors that click through your link and don't buy.
For example if 100 people click on your affiliate link and 5 of them buy (giving you a commission x 5) then your affiliate conversion rate is 5%.
When selling affiliate products, one key factor to take into account is the affiliate website's ability to convert visitors to sales - or their conversion rate.