The set of videos below describes three of the most important concepts of voltage amplifiers, that is the voltage gain, the input impedance, and the output impedance.  (There are of course many other amplifier characteristics, such as bandwidth, common-mode rejection, power supply rejection, noise characteristics)

Voltage Gain

Voltage gain is the multiplying factor an amplifier applies to the signal that is input to it. The voltage at the input gets amplified (multiplied) by the gain to give an output with a new (generally higher) amplitude. The amplification factor or gain can be stated strictly as a number (which is the factor by which the output is bigger than the input) or as a decibel. The video below goes in to all of these details

Electronic Circuits - General Amplifier Theory 1 - Gain

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Input Impedance

Input impedance is the impedance seen by a signal when it is applied to the input of an amplifier. The input impedance is important to know because, it, combined with the output impedance of the input to the amplifier result in a voltage divider across output impedance of signal source and input impedance of the amplifier. This voltage divider means the signal will be attenuated somewhat. If this is not making much sense, then the video below will help clear things up.

General Amplifier Theory (Part 2a) - Input Impedance (Zin)

Output Impedance

Output impedance is the impedance a signal sees as it is leaving an amplifier (or other signal source). If the output is connected to a load (e.g., a speaker), or to the input of another device, there will be a voltage divider between the output impedance and the load which can result in some amount of signal attenuation. The video below describes this effect in more detail.

Electronic Circuits - General Amplifier Theory 2b - Zout