These are the two methods of accounting for self-employment.
The first method is the cash
method. Under the cash method of accounting, income is generally recorded when you actually receive it, and expenses are generally recorded when you actually pay them. Most businesses without inventories use the cash method because it is fairly simple.
The second is the accrual
method. In accrual accounting, you report income in the tax year that you earned it, even if you don't collect payment until the following year. The accrual method of accounting is often used by larger businesses that carry inventory or offer credit.
Read more about the Schedule C here: https://ttlc.intuit.com/questions/1901340-what-is-schedule-c