Hi-Fi systems come in a couple of different packages. Cheap and cheerful integrated Hi-Fis are usually three-piece systems  comprising a central module that holds the CD/DVD player, stereo amplifier and FM/DAB+ radio, and two speakers.
More upmarket Hi-Fi systems designed to sound so good you’ll think the performers are playing in your listening room consist of carefully chosen, beautifully built separate pieces of equipment.
The most common of these pedigree systems comprise a CD player, stereo amplifier, tuner and a pair of stand mounted or floor standing speakers.
CD PLAYER
The CD player has two major parts inside. One is the laser mechanism which reads the digital data recorded on all CDs. The other is a digital-to-analogue converter name suggests, converts the digital data into analogue which is the language understood by most amplifiers and loudspeakers.
AMPLIFIER
The amplifier takes the low powered signals of the CD player and increases it to a level  strong enough  to power loudspeakers. The power an amplifier produces is measured in watts. An amplifier also has connections to increase the level of sound for other audio-visual equipment such as a digital radio, turntable or even a TV.
LOUDSPEAKERS
It’s true. Speakers come in all shapes and sizes. Some are ruler- flat others are round like balls. But mostly speakers come in the shape of a box. The important thing to know is that small speakers won’t sound tonally full and natural in really big rooms. And big floorstanding speakers will sound muffled in a really small room.

UPMARKET Hi-Fi SYSTEMS
Spend a little more and you’ll get a lot more intelligent design in your equipment. For example the sophisticated CD player will come in two chassis’ or even three. One houses the laser mechanism, another the digital to analogue converter and the third the external power supply.
Your money will also buy a two-box amplifier. One has the pre-amplifier, the other carries the power amplifier.
It’s a costly approach taken by all the better brands because the more separated the equipment, the better the sound.