CPM/CPC versus CPA/CPS (performance marketing)

In the case of CPM or CPC, the publisher does not care if the visitor is the type of audience that the advertiser tries to attract and is able to convert, because the publisher already earned his commission at this point. This leaves the greater, and, in case of CPM, the full risk and loss (if the visitor can not be converted) to the advertiser.

CPA and CPS require that referred visitors do more than visiting the advertiser's website in order for the affiliate to get paid commission. The advertiser must convert that visitor first. It is in the best interest for the affiliate to send the best targeted traffic to the advertiser as possible to increase the chance of a conversion. The risk and loss is shared between the affiliate and the advertiser.

From Wikipedia
For this reason affiliate marketing is also called "performance marketing", in reference to how employees that work in sales are typically being compensated. Employees in sales are usually getting paid sales commission for every sale they close and sometimes a performance incentives for exceeding targeted baselines.[12] Affiliates are not employed by the advertiser whose products or services they promote, but the compensation models applied to affiliate marketing are very similar to the ones used for people in the advertisers' internal sales department.

The phrase, "Affiliates are an extended sales force for your business", which is often used to explain affiliate marketing, is not 100% accurate. The main difference between the two is that affiliate marketers cannot, or not much influence a possible prospect in the conversion process, once the prospect was sent away to the advertiser's website. The sales team of the advertiser on the other hand does have the control and influence, up to the point where the prospect signs the contract or completes the purchase.