Z-Wave items are the first generation of these devices and Z-Wave Plus items are the newest generation of Z-Wave devices. Z-Wave Plus devices have 50% more wireless range, 250% faster processor, 400% more memory, 50% more energy efficiency and they support wireless upgrades.
Both generations of Z-Wave certified devices work together. This means that even if you have a classic Z-Wave controller or hub, it will still work to control the newer Z-Wave Plus products, and vice versa. So, you can continue to use your existing Z-Wave devices unless you choose to upgrade. For more information check out this blog post on Z-Wave Plus.