CarPool is a cross-plateform app (iOS and Android at the beginning and as soon as possible the others) that allows anyone living in a major city to find either a ride to go where they'd like (to work, to the gym, to school, downtown ...) in exchange of a few euros or to find someone who goes the same way to share your ride and save a little money on every days trips.
If you are looking for a ride: add via the dedicated app your destination and find someone nearby who goes the same way. When you have reached your destination, a few euros will be transferred to your driver's CarPool account via the app. The fare is showed before you accept the ride and it is calculated automatically depending on the distance.
If you are a driver who wants to share his ride: add via the dedicated app your destination and find someone who also needs to go there. You'll be granted with a certain amount of money at the end of the ride, transferred directly to your account.