The keys are available in three versions: linear, tactile or clicky. The first offers a smoother keystroke, while you're perhaps more likely to feel actuation with the tactile keys. As for the third version, you should get that satisfying clicking sound many people want from mechanical keyboards, as well as tactile feedback.
Both the G815 and G915 keyboards have a quintet of programmable keys and support for 16.8 million colors through the RBG lighting system. The major difference between them is that the G915 is wireless, and it uses Logitech's low-latency Lightspeed tech for minimal lag. Its battery should last for 12 days on a single charge when you have the RGB system switched on, and up to 135 days with the lights off. The G815 costs $199.99 and the wireless model will set you back $249.99.