Mild Hybrid is a hybrid where there is a small amount of assist added to the gas engine torque to help move things along. If the car is moving, the gas engine is running. The Malibu Hybrid and Civic Hybrid fall under this catagory.
Full hybrid like the Pruis the electric motor and gasoline engine work in tandem while the transmission blends the power from each source. Sometimes the car can be moving on just one or the other, but rotational motion is provided independently and blended later as needed. The "plug-in" aspect just means that the car gets a much bigger battery for longer electric operation and greater amounts of regeneration able to be captured in storage for later.
The Volt isn't a hybrid in this sense because rotational motion is never "blended" Except in one specific driving condition, your forward motion is always coming from the electric motor even if the gasoline engine is running. GM found out that above 70mph, if the battery is dead, the engine and generator spin at about the same RPM, so for efficiency reasons, it was better to lock them together. That is the only situation where the Volt can be considered a hybrid. Drop below 70mph or recharge the battery a bit (from a long downhill run) and the connection will be broken and you will be back to electric only propulsion.
When you are in your Volt with a fully charged battery, you can accelerate at full throttle all the way up to 100mph and never use a lick of gasoline. It is purely an electric vehicle at this point.
In the Plug-In-Pruis, above 45 mph, the gasoline engine kicks on regardless of battery status because the electric motor alone is not powerful enough to propel the car above that speed without assistance from the gas motor.