Earnings are the net revenue of a company after expenses and, sometimes, taxes. Also known as profit.
Corporate earnings are an important metric for individual companies and the economy as a whole, because it shows how much money has been made. Revenue is the inflow of money to a company, and earnings are the net revenue, or profit, after expenses have been taken out.
Of course there is also EBITDA, which is Earnings Before Interest Taxes Depreciation and Amortization, but this is a non-GAAP method. Earnings are usually calculated on a quarterly basis.
Earnings per share (EPS) and other ratios like PEG are important and ubiquitous tools for evaluating the quality of a stock. Earnings per share is the amount of the earnings to which each share is entitled, and the PEG ratio (Price to Earnings over Growth) attempts to reveal whether a stock is over or under valued based on the earnings growth rate.