How to find the difference between Amplifier and Receiver?
If you see an amplifier in a store for display, simply check for the features that differentiate the amplifier from the receiver. How does the front look? Does it have many switches and controls? Yes? Than it is probably a receiver. An amplifier is pretty plain from the front. Now check the back, does it have many inputs and outputs for hifi-components at the back? No? Than you have an amplifier in front of you and will need to add a pre-amp to switch between Hifi-components.
How to use a receiver or amplifier?
To use a receiver, you simply plug each device in one of the inputs of the receiver and switch between your hifi-components with a switch on the receiver´s display. If you own an amplifier and want to use different hifi components, you will have to use a pre-amplifier. A pre-amplifier has a ll those functions a receiver does, but instead of amplifying an audio signal directly to the speakers, it simply sends the signal to the amplifier. Therefore, an amplifier with a pre-amplifier has all the options a receiver does. Or in other words, a receiver is an amplifier with additional features.
Should I choose an amp and pre-amp or a receiver?
Most people prefer to buy a receiver over an amplifier, as it takes less space and includes all the needed features in one box. Also, receivers frequently have additional features, such as a tuner. Not only does this save space, but is often the more cost-effective choice. You should also not forget the visual aspect. A receiver has usually a nice looking equalizer and many buttons and knobs. The amplifier is a plain box and for many a rather unattractive object.
On the other hand, if people are looking for high-end HiFi-equipment, the difference in audio quality might lead them to choose a pre-amplifier and an amplifier over a receiver. It is assumed that dedicated components for each task will lead to a better sound quality than one box that does it all. Possible drawbacks are that one has to choose each component wisely and usually pay more. Aiming for "best sound quality" is also fairly subjective and can vary a lot on one´s ears and the individual needs.