Google AdWords

To get useful information from your AdWords campaign in your Analytics account, you must link your accounts. This is not done automatically.

Concepts

Dashboard: not very useful, a collection of shortcuts.
Intelligence: useless, allows you to define automatic alerts.
Visitors: information about visitors (browsers, capabilities, country).
Traffic sources: very important, can help you understand where your visitors come from.
Content: which pages are viewed.
Goals: user-defined goals.
E-commerce: track transactions and items sold.
Custom reports: this can be quite useful as well.
Advanced segments: crucial, allows you to set custom segments.

Only 5 custom variables. To overcome this, you can put data in the URL (maybe with virtual page views?).
50 variables are available in the Premium version of Analytics.