HDMI - High Definition Multimedia Interface.
From the name, you can understand that it is an interface. HDMI first comes about with the gaining popularity of Blue-ray DVD players and flat panel LCD/LED televisions.
In HDMI, you have 19 individual wires wrapped inside and what makes HDMI so special is that it's capable of transmitting a gigantic rate of 10.2 Gbps which should be enough to last through the burgeoning audio-visual requirements for the next 5 years at least. This by itself, is already a sure-sell feature over other AV interfaces like component video or composite video.
Another sure-sell advantage of HDMI is that it is pure digital transmission without the need for signal conversion at sender/receiver end, thus making the picturs and sounds even sharper and clearer.