ARPU

ARPU (average revenue per user) spreads a period's revenue over all active users, whether they pay or not. ARPPU (average revenue per paying user) spreads it over paying users only:

$\text{ARPU} = \frac{\text{revenue}}{\text{active users}} \qquad \text{ARPPU} = \frac{\text{revenue}}{\text{paying users}}$

The two are linked by the share of users who pay:

$\text{ARPU} = \text{paying share} \times \text{ARPPU}$

That identity is the reason to track both. ARPU is what you can put next to acquisition cost for all users, and it's the input for forecasting revenue from user counts. ARPPU tells you what a paying customer is worth, and it moves with price, plan mix (monthly vs yearly) and discounts. When ARPU changes, the identity tells you whether more people started paying or payers started paying more.

Before comparing numbers, fix three details: the period (monthly ARPU is not daily ARPU; some teams use ARPDAU, revenue per daily active user), which users count as active (the same definition as your MAU), and gross or net revenue (before or after the store commission and refunds). A yearly plan also makes ARPPU lumpy: recognize it as one-twelfth per month, or the month of a big yearly-plan push will look like a price rise.