Enhanced CPC, or enhanced cost-per-click bidding is a hybrid bidding model in AdWords that is a mix between automated bidding, manual bidding, and Conversion Optimizer. With ECPC bidding, you get to pick a unique bid for each keyword in your account, just like you can with manual CPC bidding. But with ECPC, you are allowing Google to increase your bid by as much as 30% for clicks that Google thinks are most likely to convert. At the same time, it can bid down by as much as it wants (more than 30%) for clicks that it feels are less likely to convert.
ECPC should be compared with Conversion Optimizer, which is the other bidding strategy that attempts to maximize conversions.