Pam Fletcher, as an automotive enthusiast (who is looking forward to our future of electric cars) in order to keep my brain from filling up completely, I don't keep track of all of the intricate differences between hybrid cars.
After learning the intricacies of each system, I tend to lump them into three categories for my convenience (plug-in capability being a sub-category itself).
Weak parallel hybrids; those with very minimal battery storage and no all-electric mode like the 1st gen Ford Escape.
Strong parallel hybrids; those with large battery capacity, all-electric mode, but the internal combustion engine still drives the wheels often like the Toyota Prius, Honda Insight, etc.
Series hybrids; AC Propulsion T-Zero with trailer generator (or any all-electric with a generator on a trailer for that matter), Chevy Volt, etc.
So I've lumped the Chevy Volt in the series hybrid group, although technically it can provide some power to the wheels mechanically via the internal combustion engine. With the Volt being so close to a pure series hybrid, I'd like to know why the leap to a pure series hybrid wasn't made completely? There must be one or a few solid reasons. Was it a serendipitous capability due to the packaging? What is necessary to satisfy focus group complaints? Was it to ensure a completely dead battery or charging system wouldn't side-line the vehicle? What was the thinking there?