I know both are bluetooth smart devices. I need to know whether both can be used for the same applications. If not what do they have in common and what is different about them?
A sensortag can be configured to be an iBeacon, but it is designed to be a more generic Bluetooth LE device that can be put to many other uses as well, providing many other Bluetooth services.
An iBeacon is a very specific type of Bluetooth LE device, and many types of iBeacons can only perform that one function.
Because a sensortag is so generic, it is not optimized to be an iBeacon. Its battery, for example, will not last a super long time when acting as an iBeacon.