Each taxpayer (business or individual) must figure taxable income on an annual accounting period called a tax year. The calendar year is the most common tax year. Other tax years are a fiscal year and a short tax year.
Each taxpayer must also use a consistent accounting method, which is a set of rules for determining when to report income and expenses. The most commonly used accounting methods are the cash method and an accrual method. Under the cash method, you generally report income in the tax year you receive it and deduct expenses in the tax year you pay them. Under an accrual method, you generally report income in the tax year you earn it, regardless of when payment is received, and deduct expenses in the tax year you incur them, regardless of when payment is made.
For a complete discussion refer to Publication 538, Accounting Periods and Methods.