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?

AdSense = Google fills your ad space automatically, minimal control. Ad Manager = an ad server where you manage everything yourself: direct deals, multiple ad networks competing, pricing rules, detailed reporting. Ad Manager only really makes sense once you’ve got solid traffic or direct advertisers.

AdSense is easy to operate in Google. Simply place the ads on your website and get paid whenever users visit or click on the ads.
Google Ad Manager comes handy for those publishers who require more control over their ads, such as management of the ad networks, placements, and even advertisers.
For blogs and small websites, AdSense will serve better. For larger websites with heavy traffic, Ad Manager may become useful.