This rubber thing is known as plug boot. A booted cable has the plug boot on the Ethernet patch cable end of the connector. You can find two main types of boots on the market. One is the separate boot that can be purchased individually to put on the cable, and the other is the injection molded boot to be bought together with the cable. Of course, the latter is stronger to provide support for the cable when the connector is being pulled out of a switch, patch panel or other devices. Non-booted cables will not have the plug boot as the protection, which is easier to be damaged.