A pay period is the span of work a single paycheck covers.
Common lengths
Weekly — fifty-two paydays a year. Every two weeks — twenty-six paydays, and two months each year contain three of them. Twice a month — twenty-four paydays, always on the same dates. Monthly — twelve.
Why the choice matters to cash
Every payday is a demand on the bank account, and payroll is usually the largest single one. Shorter periods mean smaller, more frequent demands; longer ones mean fewer, larger.
The two-week cycle carries a trap worth planning for: twice a year a month contains three paydays rather than two. A business budgeting on a monthly average will be short in those months unless the extra run is in the cash flow forecast.
Consistency
The period governs how labour cost is read against revenue. Comparing a three-payday month with a two-payday month without accounting for the difference makes a normal month look like a bad one.