An Integrated amplifier consists of a preamplifier and stereo power amplifier in a single chassis.
According to Wikapedia "A preamplifier (preamp or "pre") is an electronic amplifier that converts a weak electrical signal into an output signal strong enough to be noise-tolerant and strong enough for further processing, or for sending to a power amplifier and a loudspeaker. Without this, the final signal would be noisy or distorted.
There are both advantages and disadvantages to using a preamplifier/power amplifier system.
The main advantage of the pre/power system is the level of choice that you can have in choosing the correct preamp and power amplifier to meet your requirements - very much a personal mix and match scenario. You can mix valves with solid state the permutations are limitless. It also gives you greater potential for a upgrade path.
The disadvantages are, a pre power setup takes up more space as it can be a minimum of 2 boxes - even more if you mono bloc your system or go active with your loudspeakers. There is also the question of which interconnects between the preamp and power amp work the best - something that you do not have to consider in an integrated amplifier as all connections are generally made by the circuits track layout.
There is no right or wrong way to go with regards to the method of amplification you choose - it is very personal, just like choosing the other items that make up your HiFi system.
At Dooleys HiFi we have tried to bring our customers a selection of what we consider to be the best Pre-Power amplifiers available that cover a wide price range.
Musical Fidelity, Leema Acoustics, Luxman, Pro-Ject, Quad, and Rotel