AGM batteries have become very popular over the years especially for bikes due to how you can manipulate mounting positions for the battery without having to worry about the battery leaking. The battery having a Glass Mat that is being absorbed from the acid allows for that to occur. Another great benefit of an AGM battery is the amount of gas that is given off is much lesser compared to a conventional battery making it a slight safety improvement.
How to Test a Motorcycle Battery : Weekend Wrenching