A transistor is a three-terminal semiconductor device whose defining feature is that a small electrical quantity applied between two of its terminals controls a much larger current flowing between two others. That control relationship — a weak input steering a strong output — is the entire reason transistors exist and the prerequisite for amplification.
A Diode is a one-port device: once you know the voltage across it you know the current through it, and that is the whole device. There is no way to make a diode’s current depend on a separate input. A transistor breaks that limitation. With three terminals, one terminal (or the voltage/current at it) acts as a control knob on the current between the other two. A tiny input wiggle produces a large output wiggle, and the ratio of the two is gain.
The third terminal lets one part of a circuit control current in another — the prerequisite for amplification or switching.
Why a controlled current source is so powerful
If a transistor can make its output current track an input voltage, then feeding that current into a resistor turns it back into a voltage — and if the ratio is chosen well, a much bigger voltage. That is a voltage amplifier. The same controllability builds oscillators, mixers, current sources, and every active signal-processing block. Without a three-terminal controlled device you cannot make energy from a power supply do useful work on a weak signal.
The same device used differently is a switch. Drive the control terminal hard one way and the output conducts fully (a closed switch); drive it the other way and the output is off (an open switch). That binary behaviour is the basis of all digital logic — every logic gate, memory cell, and processor is built from transistors used as switches. [Background from general knowledge, not the source PDF: in this course the transistor is studied as an amplifier; the switch role is the same device biased into its hard-on and fully-off states.]
Two families
The two transistor families studied in this course are the BJT (bipolar junction transistor) and the field-effect transistor, whose dominant member is the MOSFET. They reach the same end — a controlled current — by different physics. In a BJT a small base current controls a large collector current; charge carriers of both polarities (electrons and holes) take part, which is what “bipolar” means. In a FET a gate voltage sets up an electric field that controls a channel; only one carrier type conducts. MOSFETs dominate modern electronics, especially digital logic, because they are tiny, draw no steady control current, and pack by the billions onto a chip. BJTs survive in analogue niches where their higher transconductance and device matching matter.
Other families exist — JFETs, MESFETs, HEMTs, IGBTs — but the BJT and MOSFET cover essentially all of what you need here.