iTunes provides both sale and rental options for titles on their storefront. Any customer can access the iTunes storefront for free and make purchases.
If a customer rents a title, the customer has 30 days to start watching the title. Once the customer starts watching a title, the customer has a 24-hour window (US) or 48-hour window (elsewhere) to finish watching the title.
If a customer purchases a title, the customer can watch the title indefinitely via the customer's iTunes account.
To view the iTunes storefront, click here. The iTunes storefront can only be accessed via the iTunes app.