Generally Accepted Accounting Principles (GAAP) Revenue is the income earned by a company through its regular business activities. It includes items such as sales of goods and services, interest, rental income, and royalties. GAAP revenue includes only income that is both realizable and earned during the period covered by the financial statement; it excludes unrealized items such as a gain on sale of an asset. Ultimately, GAAP revenue provides investors with reliable information about a company’s performance and prospects for growth, which is why it’s so important.