A surround sound system turns a television into a home theater. Without it, you can never approach the audio experience intended by modern-day filmmakers (or even some filmmakers from the 1950s). Sure, your HDTV can probably fake surround, but it’s not going to sound like the real thing.
That real thing requires a receiver (sometimes called an amplifier), and several speakers.
Receiver
can buy everything in a single package, or get the receiver separately from the speakers (you should purchase the speakers together). You’ll get a better deal if you buy everything in one package, although hardcore audiophiles usually prefer to buy them separately.

Read more here: http://www.techhive.com/article/257229/what_should_i_look_for_when_buying_a_home_theater_sound_system.html
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It is amazing when you think about all the sophisticated display technology that goes into a modern flat-screen TV. And then you get two tinny, 10-watt speakers to go with it. That’s fine for TV sitcoms, but it won’t cut it for movie effects and soundtracks.
That’s why you want a separate sound system. I’ll run through your best options from cheap to expensive. The good news: Even the least-expensive options will dramatically improve the sound of your new TV.
Read more from Kim Komando on that topic here: http://www.komando.com/buying-guides/12663/buying-a-home-theater-sound-system-2/all
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