Home Theater In a Box, or HTiB is a 5.1 or 7.1 speaker surround sound, virtual-surround, or soundbar digital audio system, pre-packaged and shipped in one box. It is designed to be connected to your high definition TV or video projector and multiple A/V source components such as your Blu-ray Disc player (unless you buy a system with an integrated Blu-ray Disc player), video game console(s), and/or HDTV tuner/ DVR to re-create that spine-tingling, larger-than-life sensation of catching a movie at the cinema -- but scaled down to fit the smaller confines (and creature comforts) of home.
See the 7.1, 5.1, 3.1, 2.1, and 1.1 links across the top of the page? The number before the dot represents the number of speakers that will need to be set up throughout the room, and the 1 after the dot tells you there's a separate subwoofer included that sits on the floor and produces just the low frequency bass effects.
In general, the higher the number of loudspeakers involved, the more cinema-like the potential audio performance, but also the more time, difficulty, or expense may be incurred during proper setup and installation.
Large and medium size rooms will usually sound best with a 7.1 or 5.1 speaker surround sound system, while smaller rooms may not be able to handle more than a 2.1 or 1.1 speaker soundbar + subwoofer system.
Before you settle on a specific home theater system in a box, be sure that it's got more than enough inputs and outputs (such as HDMI, digital audio, component video, USB, iPod/iPhone connectivity, headphone jack, Zone 2 speaker capability, etc.) to handle all of your current and future source components, and adequate amplifier power and speaker quality to fill up your particular room with realistic multi-channel digital surround sound audio effects. Also, be sure to plan ahead of time where you want to place all the components and for wiring each of the speakers.
Depending on your particular situation, you may also need to separately buy some furniture for supporting and/or mounting your flat screen television or video projector, components, speakers, and don't forget about comfortable seating. You may also need to purchase some cables, speaker wire, and a surge protector.
Lastly, if you don't have all the knowledge, experience, skills, and/or tools to set it up properly yourself, get some help.