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What is the difference between operating a transistor as a switch and as an amplifier?

When a transistor is used as a switch it is fully turned ON and OFF by a control signal usually applied between the base and the emitter. This causes the collector to emitter resistance to drop to a low value.. much like a simple electrical switch. Common electrical components such as lamps can be connected in series with the power supply voltage and the collector and emitter of the transistor.
This allows a small control current applied to the base of the transistor to control a much larger current through the lamp.

See here for more: Transistors as switches

When used as an amplifier the transistor operates in a linear region between cutoff and saturation (Not fully on or off).
When the base current is set to operate the transistor in this region, it can be used to amplify a input voltage or current in a linear fashion.
This usually requires additional resistors to set this operating point.
Different configurations that be used to make a current amplifier (no voltage amplification), or voltage amplifier depending on the application.

See here for more: Basic Transistor Amplifiers

A semi-conductor can made from silicon, germanium (and a few other elements) by adding small amounts of other elements in a process called “Doping” (no relation to illegal drugs).
Two different types of semiconductors can be made in this process (called P and N type).
A semi-conductor can be made to act like either a conductor or an insulator depending on the applied voltage polarity.
P and N type semiconductors are combined to make diodes, transistors, scrs and other electronic components.

See more here:
Doping to make semiconductors