Apparently, here are the full stats:
Cycle through resist types. While healing, provides you and your target with a constant 10% resistant to the selected damage type.
+50% ubercharge rate
-66% overheal build rate Ubercharge provides 75% resistance to a selected damage type. You are healed for 25% of the matched incoming
damage on your heal target.
What happens is, by toggling alt+fire, you can switch the type of resistance the target has. Refer to @Kenogu answer for resistance info.
Ubers are divided into 4 sections, giving you 4 SEPERATE ubers. Each "uber" takes roughly 20 seconds to charge. You can control when you want to use your ubers, so let's say you use the first two ubers, then save the rest for later.
The uber itself grants 75% resistance, as mentioned, to a specific type. It is a resistance to ONE type, not MULTIPLE types. Also worth mentioning that it is not an uber, just a resistance to a certain damage type. Another thing as well is that while in uber, you cannot change your resistance type.
Here's an image depicting the uber and medigun in action: