Receivers and amplifiers are both devices that are used to amplify audio signals. However, there is a significant difference between the two. A receiver takes an incoming audio signal and then amplifies it before sending it to the speakers. An amplifier, on the other hand, does not have this ability to receive an incoming signal. Instead, it simply takes an existing audio signal and amplifies it.
So, in short, a receiver acts as a sort of hub for your audio system, while an amplifier just boosts the signal that is already present.