Net:

"A network representing many Web sites in selling advertising, allowing advertising buyers to reach broad audiences relatively easily through run-of-category and run-of-network buys.
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Ad networks act as a broker between merchants and affiliates. They have all the offers that you can use to earn commissions.

Hi Gdiddy,

An ad-network is basically a company that is in charge of making the best connection between an advertiser and a publisher; the ad-network usually buys the unsold traffic from the publisher and maximizes the profit by running a suitable campaign on it.

One of the differences between an ad-network and an affiliate network is that an ad-network has better control over the traffic by having the most possible direct relationship with the publishers.

Using an ad network is important both to the advertiser and to the publisher;
On the publisher side: There is a bigger chance to profit from the unsold traffic, since the ad network is buying it and using it with traffic from other publishers- the volumes the ad network is offering assures the publishers side that there will always be someone looking to buy a campaign and pay for traffic.
On the Advertiser side: It’s another very important online-marketing channel, it’s a chance to increase conversions and impressions, and the large volumes the networks work on assures the advertisers that they have no need in worrying about the campaign and constantly preform media buys- the networks they work with are all taking care of it.