What most people call an amplifier, is actually called an integrated amplifier. Integrated means that all of its amplifying functions are contained in one box.

Typically, these amplifying functions include:-
RIAA amplifier, also known as a Phono Stage, specifically for use with a turntable.
Control amplifier, providing Volume, Balance, Source selection etc.
Equalisation amplifier, also known as Tone Control.
Power amplifier which drives the speakers.

An Integrated Amplifier combines all of the low level and high power elements and of course, it's got two of everything because it's stereo!

It's also possible for each of these functions to exist in its own box and this commonly occurs in high-end systems.

The most common 'split' is to put the sensitive, low power bits into one box, the Pre Amplifier, (sometimes called the Control Amplifier) and all the heavy, high power bits into another, the Power Amplifier, which handles all the Watts.

The most minimal Pre Amplifier (or Preamp) provides three fundamental and critical functions:-
Input Selection, Volume Control and Line Drive which provides the output. Although each of these functions is very simple, the whole system's performance depends upon them being done properly.

These days, many preamps do not have the RIAA (phono stage) included for two reasons:-
Some people are so serious about vinyl they want a special phono stage in a separate box.
Some people don't care about vinyl at all.

Of course, there are also preamps that have the phono stage built in so there's at least three ways to go there. Classic preamps (from the Golden Years) will usually be fully featured with all the bells 'n whistles such as tone controls, filters, tape dubbing etc. while the more contemporary, minimalist types just provide the critical functions.

What is RIAA? It stands for Recording Industry Association of America and it's a standard that enables LPs to be 'Long Playing'. Before this standard was established, the maximum playing time was about three minutes per side, a factor which completely changed the shape of the popular music industry by forcing all artists to get their songs done within in that time constraint.

The advent of 'Microgroove' records meant that Long Playing records, LP's became possible. Making the grooves smaller, and more tightly packed, could only be achieved by 'precompensation' which, in the simplest terms, turns the bass way down because bass requires much bigger grooves than treble does.

RIAA specifies the equal and opposite curve of this precompensation to equalise the bass and treble on playback. The phono stage applies this curve to the signal from your turntable in addition to amplifying its tiny signal about 1000 times to bring it up to the same level as the other 'line level' sources like CD, Tuner etc. The quality of this stage determines the quality of sound from vinyl just as much as does the quality of the entire turntable /arm / cartridge assembly which explains why many of the more serious vinyl enthusiasts prefer to use a separate, special quality component.

So, in conclusion, a preamp accepts signals from line sources such as CD, Tuner, Tape and sometimes low-level sources such as Turntable and Microphone if applicable. It provides a control panel for all of those sources and sends them to a Power Amplifier whose job is to provide the energy required to drive speakers. Some integrated amplifiers also provide a pre/power split facility with either a switch or some links on the back that enable users to access the preamp and power amp functions separately.