DVD Player – A device for playing video DVDs.

Surround Sound Decoder – This converts the surround sound information encoded on the DVD disc into the six different channels of sound to feed a surround sound speaker system. Depending on the home cinema system, the decoder may either be inbuilt into a DVD player, in an external amplifier unit, or even in a standalone decoder unit (usually only on high end systems).

Amplifier – Accepts audio input from the DVD player and feeds the speaker system.

5.1 Speaker System – So called because it consists of five speakers and a subwoofer. This set of speakers is positioned around the room to replicate the surround sound system used in your local cinema.

A complete home cinema system package usually consists of DVD player, decoder/amplifier and 5.1 speaker system.

Separate Component Systems

Amplifier -The type of amplifier needed in your setup depends on whether your DVD player has a surround sound decoder (processes the audio information into the separate channels involved in surround sound). If your DVD player doesn’t have inbuilt decoders, then your amplifier must have them.

The decoder and amplifier can be bought separately, but this is more expensive, and most people go for a combined decoder/amplifier.
Some amplifiers also include a radio tuner and are known as receivers.