A potentiometer (sometimes just a "pot") is a mechanically adjustable resistor.
In general, a resistive strip of material is exposed with a terminal at either end. Then, a wiper is mechanically moved to make contact somewhere along that strip, with one terminal on the movable wiper. Overall, the potentiometer has three terminals, and the schematic symbol represents the idea of a wiper moving across a resistor:
There are many variations of potentiometers:
Sometimes the wiper movement is accomplished with a rotary motion, like a knob, and sometimes with a linear motion, like a slider.
Sometimes the wiper motion is basically linear with resistance. Other times, the strip is designed to give a certain profile of position versus resistance (for example, a logarithmic scale).
Sometimes the potentiometer is hidden in a place that is not adjustable to the end user of the electronic device. In this case it may be adjusted by manufacturing and test personnel, and is sometimes called a "trimmer." In other cases, the potentiometer is a user-adjustable feature.
Sometimes, there are multiple mechanically connected potentiometers, where a single knob controls two (or more) independent potentiometers. For example, for left and right channels of an audio signal, there might be two independent electrical potentiometers connected to a single mechanical knob.
The potentiometer can be used as a two-terminal device. In this case, it's just an adjustable resistor.
We can pick the wiper terminal plus either of the outside terminals. Either is fine, so we can choose which is best based on our desired mechanical layout.
The remaining terminal will be left open. Since no current can flow, all the resistive material outside the range of the two selected terminals has no effect.
The potentiometer can also be used as a three-terminal device.
In this case, the wiper effectively forms a voltage divider, splitting the total resistance $R$ into two pieces, $k R$ and $(1-k) R$ if $k$ is the position of the wiper from 0 to 1.
If the current drawn from the wiper terminal is kept small, and both ends of the resistive material are at known voltages, then the overall effect is that turning the potentiometer knob selects a voltage in between the two extremes.
Potentiometers are mechanical devices and rely on good contact between the movable wiper and the resistive material.
As a result, potentiometers wear out. They can suffer from corrosion, or even simply a loss of spring pressure over time. They're also especially noisy when they're being moved.
For these reasons, potentiometers are less common than they once were. In trimmer and other manufacturing adjustment situations, they're being replaced by digital adjustments where possible. And in user-facing scenarios, like knobs or joysticks, optical or magnetic solutions are becoming inexpensive and reliable enough to take over.
In the next section, Resistors in Series and Parallel, we'll look specifically at building circuit networks out of resistors and how currents and voltages behave as multiple elements are combined.