A home theater system is a combination of components designed to provide a theater experience at home. This typically includes surround sound speakers, a centralized receiver, a TV/projector, and some video source like dish, cable, or Blu-ray. The main component of a home theater system is the surround sound system which provides a 3D sound feel of theaters by placing speakers both in front, behind, and above.
The simple home theater surround sound system is comprised of 1 central front speaker, 2 front side speakers, 2 rear side speakers, and a subwoofer. This is called the 5.1 channel configuration, because there are 5 ear-level speakers, and 1 subwoofer for low sound frequencies and bass. When you see the word channel, just think "number of speakers". There are many variations to choose from, and those variations typically come in a representation like 5.1 channel, 7.1 channel, 7.2.4 channel, and so forth. Let's break down what each part of the channel means.
1. 5.x.x, 7.x.x, 9.x.x - What does the first part of the channel number mean? It tells you the number of ear-level speakers the home theater system has. 5.x.x means there's 5 ear level speakers. 9.x.x means there's 9 ear level speakers.
2. x.1.x, x.2.x - What does the second part of the channel number mean? That's the number of subwoofers. Subwoofers give you bass effects and will rumble your floor during a movie. Any home theater system must include at least 1 subwoofer for a true home theater experience.
3. x.x.2 x.x.4 - What does the third part of the channel number mean? That's the number of "height" or "presence" speakers, also called the Dolby Atmos speakers. These speakers are either mounted above pointing down, or they are placed on the ground pointed up to bounce the sound off the ceiling and create a 3d sound effect. This is what vastly improves the home theater surround sound feel by making you feel like you're present in the movie.