The term "revenue" (aslo called "sales") is an accounting concept that refers to increases in a company's assets or decreases in its liabilities as a result of its primary business activities, such as selling goods or providing services. If ABC Corp. is in the business of selling widgets and in the year 2012 it sells 1000 widgets for $10 each it will have revenue of $10,000. The opposite of revenue is expenses, with are decreases in assets or increases in liabilities resulting from the company's business activites. A closely related concept is that of gains, which are increases in assets due to "incidental" or "peripheral" activites, meaning those other than the major business of the company.
Revenue is often described as the "top line" on the income statement of the company, as opposed to the "bottom line" which is net income.