What is inside a sound bar?

A marketing device, for people that don't care about sound, but need to be sold something that's about sound.

The device uses several cone speakers in a line with a processing device to create the impression of surround sound from a flat bar instead of speakers around the room. The 'duct tape' version of a 5.1 surround system.
So it is a cheap substitute for a true 5.1 system.
When you want something more than just the TV speakers, but can't afford a real receiver with the required speakers.
Now some of these things cost two grand. they are still a slapped together cheap substitute for a real surround system.

While it represents a compromise from a dedicated multi-speaker surround setup, high quality sound is obtainable from soundbar type products, particularly if you use them with a quality subwoofer. Look for the higher end Yamaha or Definitive Tech models.

It is possible to get better sound for equivalent or less money than a soundbar, but they are an elegant solution for people who don't want electronic clutter in their living spaces.

I have a 50" Polk Audio Sound Bar. I'm very impressed with the dynamics and center and main channel and two side speaker simulations. I'm not sold on it reproducing the rear channel information. If you are looking for a very good center and main setup with a good subwoofer it would be equalivent to the Dolby 3 -1 setup of 2 main speakers, a center channel and subwoofer or two. This is just for watching movies. It blends in perfectly with my 50" monitor. I do miss the information from the rear channels. The manual says sit in front of the sound bar and I have yet to do that. Call it cheap, what ever but The Polk Sound Bar is seems is a pretty good value with what it does. It's not going to be for everyone because of the lack rear channel information.

It's all cone drivers and they have another model that is SDA (Stereo Dimension Array) but it appears to be derived from playing with the phasing of the drivers to give you an illusion. There are some electrostatic center channels and some that contain electrostatic type properties. It is an inexpensive means of obtaining surround sound effects.

Definitive Techonlogy makes a small system with sub woofer, center channel, fronts and rears that is pretty darn good. I heard it in an audio store recently and it was dynamic as all get out and did a very good job of giving you that dimensional sound that people want in home theater and it starts around $600 for the set.
No sound bar just small center channel, mains, and rear gave excellent clarity and dynamics with a Cambridge surround receiver. The rear channel effects was very good.

I won't say I would pay the $500 for this sound bar but I purchased mine at a flea market in the box for no more than a prayer. I'm satisfied with what it does but I sit to the side of my screen so no matter which setup up I use the only thing I'm missing is the rear channel information. I'm satisfied with the Polk Audio 50 Soundbar and as I said the Dynamics with my Onkyo receiver with Audssey sound equalization is absolutely wonderful. We watched 2012 with this configuration and we were all on the edge of our seats. Tomorrow I will watch the Mechanic. I'm using an Oppo DVD player, 15" Dalquist active sub, PolkAudio SoundBar 50, Onkyo TSX 306? (80/140 watt/channel) receiver, Direct T.V., and that's it. My wife always ask why I have to keep so much noise. I respond with a stare and respond, "It pleases me!" LOL.

Unsound - maybe it's a substitute for a 5.1 system for those people who don't have room or interest, and have other things to do like read a book, play with their kids, help someone out, build something or whatever, rather than obsess about which cable to buy, what power chord(sic)is the best, what speaker will "blow away" another, the nuances of image depth or some other audio neurosis that appears to take so much time for so many people (that maybe could be used to play with their kids, or build something......)

To the OP - it's a bunch of (usually) cheap speakers in a box with some set up to achieve a (usually) poor psychoacoustic effort of having a 5.1 system. Some are alleged to be OK at their purpose (I haven't heard them), but they typically have a very small sweet spot