Soundbars came out of nowhere and have domintted the home AV market. The combination of thin-sounding flat-panel TVs and consumers looking for a simple solution that doesn't require much wiring were all it took for the new style of speaker to gain a foothold.
Some soundbars are self-contained, some have separate subs and surround speakers. All are designed to fit underneath you current TV with as little setup and as possible. Plug, play, watch your TV. They won't replace 5.1 or 7.1 systems when it comes to enveloping you in surround sound. But for more casual TV viewing they are perfect, especially if space is at a premium in your home.