The project's website is http://www.go-jek.com/ (you can also download the app from there).
Orders are placed using a smartphone.
Payment can be made in cash or from your app balance (which can be topped up through a local bank).
Of course, rides are usually cheaper than taxis and faster because they avoid traffic jams.
Some interesting aspects: the service can be used not only as a taxi but also as a courier service or a means to have food delivered from restaurants.
Additionally, drivers can assist with shopping.