In the dinosaur age (metaphorically speaking) entertainment technology focused on connecting your VHS and Beta playback units using an RCA cable. These are the yellow, white, and red circular jacks representing video, left stereo-audio, and right stereo-audio respectively. Today, RCA is still used but the trend is moving towards the HDMI cable. There has been a lot of buzz about this.
The reason is that HDMI carries both very high-speed video plus surround-sound audio signals all in one compact wire. HDMI cables are designed to support the extremely high data rates needed by today and tomorrow’s entertainment technology. Say good bye to messy wire-balls living behind your TV and say hello to fast, reliable, organized, connections. If you’re trying to identify the HDMI jacks on the TV, they look like a wider, flat, USB jack.
In addition to RCA and HDMI, modern HDTV’s may have Component, VGA, DVI, and USB cable jacks. Component is very similar to RCA but there are multiple wires for the video signals. There is one wire for each color type. Media enthusiasts swear the picture quality is better with the Component cable than with the RCA cable, but some people never notice a solid difference and don't think they are worth buying. Plus, Component cables can be more expensively priced.
The VGA and DVI jacks are for connecting your computer and using the TV as a computer screen. Their necessity depends on whether your computer’s video card uses VGA or DVI. Some computers today actually provide HDMI output for both video and sound and there also exists DVI to HDMI adapters. VGA is still-used but is an antiquated cable connection, while DVI is contemporary.
The USB jacks are there so that you might be able to directly connect USB sticks or digital cameras to view pictures and movie clips. They can sometimes also serve to connect diagnostic tools to the TV and perform firmware upgrades.