The amplifier forms the heart or engine room of any system, and can take many forms. Integrated stereo amp, receiver, Audio Visual receiver, power amplifier etc.
It is important to remember that an amplifier is much more than a simple switching device. It has a real and very definite impact on the performance of your system. Poor quality amplifiers can sound thin, lack any attack and dynamics and are uninvolving.
While it is reasonable to expect that the better the amplifier, the more it will cost, there have been a number of companies, NAD for example, who have shown that you can buy a quality amplifier on a budget.