Difference between Google Adsense & Ad Manager

I want to know about Google AdSense and Google Ad Manager, I know something about Google AdSense but what is the purpose of Google Ad Manager.

Its mainly useful for managing and optimizing ad inventory across different demand sources, especially when you have more control over how ads are served on your site so you have to learn how to use it too

AdSense is mainly for monetizing your website with Google's ads. Google Ad Manager is more like a system for managing and selling your ad inventory, giving you more control over how ads are served and where they come from.
are you looking for more control over your placements, or trying to bring in multiple ad networks in addition to google?