This is kinda off-topic for stack overflow, but what the heck.
Amplifiers do exactly what you'd expect - they amplify. If you have a binary signal (ones and zeroes) with some noise, the amplifier will amplify the ones and zeros, but also the noise. Amplifiers just increase the voltage of whatever it is that they receive.
On the other hand, repeaters will first decode the signal and extract only the data. They will then regenerate and resend it in mint condition.
It's like the difference between a printer (repeater) and a photocopier (amplifier). The printer makes exact copies of its input (a digital file). While the photocopier always generates slightly nosier versions of its input (the document being scanned). A signal passing through multiple amplifiers is like a document being copied over and over using copy machines. Eventually, it will be unreadable.