There are several ways to make money from your website ranging from affiliate sales, advertisement sales, link sale (frowned upon by Google), paid reviews, and many more. Advertising networks are in essence a middleman between the available ad space you have carved out on your website and potential advertisements. You can surely try to sell your ad space directly to advertisers, and you can do it successfully. The cost to you of doing this is your time, and possibly the cost of maintaining ad software. Or, you can simply sign up with an advertising network, put their code in place, and voila! Ads will start showing on your site.
The ad networks typically offer you paid per impression (CPM) ads; banners of different sizes, pops, etc; and pay you a percentage of what the advertiser is paying them. Their cut represents their cost of running their program — sales, software, hardware — and their profit. Of course each network offers a little different payout percentage, and the payouts can be negotiated depending on the size of your inventory.
Below are several ad networks that we have taken the time to research, with a short description and user reviews where given:
