Amplifiers are first introduced in Network Analysis. There, they are treated as a "black box" whereas in Electronics I, we start to see what actually is in an amplifier.
Probably the first major rule for amplifiers is we can never get something for nothing. To get ANY gain, we need another outside power source. Probably the second most important rule is all amplifiers have a gain-bandwidth constant, meaning the gain times the bandwidth is a constant. Therefore, the higher the gain is adjusted, the smaller the amount of frequencies the amplifier will amplify.