Truecaller is an app developed by True Software Scandinavia AB, a private company in Sweden.
It runs in various os’s such as Android, BlackBerry OS, Windows Phone, iOS and many more.
It has a globally created integrated caller id service. It uses crowd sourced data, that’s why they can work in countries like India where Public data is not available to everyone. As a matter of fact it violates some privacy issues of the public in a clever way. However, anyone, who is not interested, can unlist his or her number from this service.It doesn’t work most of the time. And even it worked good, I wouldn’t be recommending this to anyone.