WARNING: This tool will be discontinued on May 4th 2020
Fabric is a suite of tools developed by Twitter to help mobile developers improve their apps. It includes Crashalytics for crash reporting and Answers for analytics.
It became very popular with mobile developers as it had a simple and attractive user interface, and provided enough information to give great insights into app performance.
Google has acquired Fabric and is sunsetting it in May 2020. We'd advise you take a fresh look at the analytics tool space and pick new crash reporting and analytics tools that work for you.