Revenue is the amount earned from a company's main activities such as selling merchandise or providing services.
A gain results from a peripheral activity, such as selling the old delivery truck. A gain is the amount received that is in excess of the asset's carrying amount (book value). For example, if the company receives $3,000 for the truck, and its carry amount was $600, the company will report a gain of $2,400.
Income is sometimes used instead of the word revenue: some people refer to the rent they receive as rent income. Generally, accountants use the word income to mean "net of revenues and expenses." For example, a retailer's income from operations is sales minus the cost of goods sold minus operating expenses.