HDMI cables transfer the audio and video signal digitally. HDMI cables are used to connect an audio video device with an HDMI cable output to many devices (Plasma, LCD TV, Surround Sound Processor or projector) with an HDMI input.
HDMI cables used to be graded by the HDMI specification they had been tested to, for example HDMI 1.3 or HDMI 1.4, cables are now graded into categories, standard speed or high speed depending upon the data rate they can support.
HDMI high speed cables can support higher resolutions beyond 1080p, eg 1440p and 2160p as well as multichannel high definintion audio.
Most consumers now require a high speed HDMI cable to support features such as 3D over HDMI.