Generally:
 

• AdSense is a publisher network that allows almost any publisher to signup. Google AdWords is the biggest buyer network on the AdSense network.
• Doubleclick is subsidiary of Google, which develops and provides Internet ad serving services. Their biggest products are DFA (Doubleclick for Advertisers) and DFP (Doubleclick For Publishers).
• AdMob is essentially a mobile ad network also owned by Google. They allow app developers to monetize their apps and also promote them.

In layman's terms:
 

• AdSense is where you make money for your blog or website.

• Doubleclick is a big advertising company that works with the biggest advertisers and publishers.
• AdMob allows mobile developers to make money by selling ads via their SDK.

Adsense is essentially for small to medium sized publishes looking to monetize their website. Doubleclick's publisher products interface with AdSense directly e.g. via backfill.

Roughly you should think of these products in Web vs Mobile World. Here is a simplistic view.

 

Note how AdMob is both acting as a publisher ad network and also a buying ad network.