Average Revenue Per User (ARPU) is a measure most often used by telecommunications companies, although it is also sometimes used by others in similar industries. It states how much money the company makes from the average user. This is the revenue from the services provided divided by the number of users buying those services.
ARPU is important because it provides a breakdown of what is driving revenue growth, and it also gives some indications of what is driving margins. Growing by increasing revenues from users tends to be better for margins than increasing revenues by increasing the user base, as the latter incurs additional costs.
ARPU growth can also indicate how successful a company is being in moving users to new services (e.g. pictures messaging, data connections etc.) that are regarded as strategically important and an indicator of how margins will fare (newer services tend to be higher margin). However, companies often disclose the composition of their revenues streams separately in any case.