More, more, more about placements
Placements allow you to create groups of ad units and can help to streamline how your ads are trafficked, make reporting easier, and help optimize CPMs:
You can target placements.
Instead of targeting 15 individual ad units you could create a placement and target the placement. For example, if you sell the ad space on your homepage as a package to your direct advertisers, then you can create a homepage placement - one that consists of all of the ad units found on your homepage. Now, instead of trafficking your advertisers to each ad unit on your homepage, you can instead traffick that advertiser to your homepage placement. This minimizes the risk of making mistakes when trafficking your ad campaigns.
You can report on placements.
You could, for example, group all your leaderboards or all the ad units on your homepage into a placements and report on its performance.
Making placements targetable for Google AdWords buyers.
In addition you can also make placements targetable for Google AdWords buyers. If you choose to do so Google AdWords buyers will be able to directly target your inventory with their campaigns.
To learn more about creating placements, watch the following tutorial: