Adsense - An advertising network owned by Google and used by website publishers to monetize their websites.

Analytics - A great tool to track website visitors and all kinds of stats. Also owned by Google.

Adwords - One of the most popular pay-per-click advertising platforms on the planet (well until Facebook showed up). Works in tandem with Adsense (and Google's Search Engine) to provide advertisers with targeted traffic.

Adsense is for publishers
Adwords is for advertisers
Analytics is for tracking visitors.