Basic ROI and Conversion Tracking
Conversions can be sign up, sales, leads or pageviews

ROI is ROAS – Return on Advertising Spend
The Report Center can show you Profit (Sales rev less adwords costs)
Divide by Adwords Costs to get Adwords ROI
Cost per Acquisition (CPA) is a popular alternative
Conversions happen on sale completion, lead generated, downloaded PDFs, newsletter [...]