HDMI extenders transmit audio and video signals beyond the limits of a standard HDMI cable using different mediums like Cat5e/Cat6, fiber optic, or wireless signals, allowing for extended transmission distances with features such as video resolution support (up to 4K), audio format compatibility (like Dolby TrueHD), plug-and-play installation, and power over cable (PoC). Key specifications include the transmission medium, maximum resolution, distance, power requirements, and compatible audio formats.