HDMI extender can be wired over rj45 or wireless over wi-fi connection. With wired connection the HDMI signal is transfered to rj45 connector and over at least cat5e ethernet cable it travels a longer distance (over 10m is not a problem), then the signal is transfered back to HDMI. Such a video signal transfer can be named also HDBT or HDBaseT. Devices which enable such transfer of uncompressed HD video signal over an ethernet cable are called HDMI extenders.
Another way of transfering the uncompressed video HD signal is wirelessly over a wireless HDMI extender, which works at wi-fi frequencies. These devices are a bit more expensive, but they enable transfer of HD signal up to 100m far in the opened space.