Fusion is a digital voice mode developed by Yeasu. It is very ham friendly and very easy to use and get on the air quickly. Rather than recreate information that already exists, I will direct you to the site HamOperator in the links below. There is a ton of great information that can be found on that site.
One item that I have heard come up rather frequently regarding Fusion is the use of hotspots. With a hotspot capable of Fusion, it is important to note that you can't access Fusion Wires-X rooms with said hotspot. There are open source Fusion rooms available on YSF reflectors, but WIRES-X is a system run by Yeasu.