In the simplest terms, the difference between cash and accrual accounting depends on when the income and expenses are recorded. Organizations that use cash basis account recognize income and expenses when money changes hands. Organizations that use accrual basis accounting recognize income when a customer is billed and recognize an expense when a bill from a vendor is received.
For tax reporting purposes, an organization usually adopts an accounting method in the first year of operations and uses that method to determine taxable income. If an organization wishes to change its official accounting method, consent must be obtained from the IRS by filing form 3115.
https://www.irs.gov/forms-pubs/about-form-3115