The mobile app differs from the desktop app in terms of features. Here is a feature comparison between the two apps:

FeatureDesktop AppMobile App
Track time on Tasks and ProjectsYesYes
Website and Applications TrackingYesNo
ScreencastsYesNo
Keyboard and Mouse Activity TrackingYesNo
GPS TrackingNoNo

For details on how to install and use the mobile app, see this article.

Note: If you’re going to use both the desktop and mobile app for the same company account, you need to make sure you don’t use them simultaneously. You’ll need to close Time Doctor on one device and then open it on the other because running them both at the same time will affect your reports.