Pre-Amp: switches input sources to output sources and includes volume control. A true pre-amp rarely includes a radio tuner.
SSP (Surround Sound Processor): Includes circuitry for processing signals; eg decoding Dolby Digital or DTS signals or applying 'matrix' processing like DPLII/IIx to analog signals or 2 channel digital signals.
Pre/Pro: The combination of a pre-amp and processor. The most common form found today. Also includes a radio tuner.
Integrated amp: pre-amp + power amp (no radio tuner)
Receiver: Combination of pre-amp, processor, and radio tuner. The all in one box that so many of use instead of separate pre/pro and amp.
Amp: Amplifies the signal sent it from a pre-amp, or pre/pro or a receiver when connected using the pre-outs of the receiver.