I'm trying to learn something about iBeacon and I have a question: As far as I understood Apple provides API in order to develop iBeacons app since IOS7, but for android how does it work? The only thing I found is that It works only from version 4.3 (Is it correct?) But are there any sdk or library to use?
Yes, you can use the open source Android Beacon Library, which gives Android the same basic capabilities to detect and transmit as beacons as provided by CoreLocation on iOS devices. This library is designed to be vendor neutral, and works with a wide variety of beacons. There are also a number of proprietary Android SDKs offered by beacon manufacturers, some of which harness special features that only work with those beacons.
The main thing to understand on Android is that while 4.3+ devices can all detect Bluetooth LE transmissions, there is no native beacon framework, and working with beacons typically requires quite a bit of logic beyond reading the Bluetooth LE packets they send out. As a result, Android beacon apps typically bundle a small library like mentioned above with the app to provide beacon detection and/or transmission capability.
Full disclosure: I am the lead developer for the Android Beacon Library.