An inductor is an electronic device which consists of a coil of wire which may have a metallic or ferrite core. If it appears to have no core or has a core with no magnetic properties (like a plastic spool), it is considered to have an air core. The core material will greatly affect the value of the inductor. If everything else stays constant, increasing the number of turns of wire around the core, will increase the value of the inductor.

The unit of measure is the 'henry', but since that is such a large value of inductance, the value is usually stated in millihenries or microhenries. One henry is equal to one thousand millihenries or one million microhenries.

This is the schematic symbol for an inductor.

Examples:
The following few images are example of common inductors. The first one is the type that you might find at the output of an amplifier (internally). It's used to compensate for highly capacitive loads and make the amp more stable (less likely to oscillate). This is essentially an air core inductor. The resistor in the core has little or no effect on the overall inductance.

This is an inductor that you might find in a low power circuit. This is a 1000 microhenry inductor. These are typically used to remove high frequency noise to prevent the circuit from malfunctioning. This is a ferrite core inductor. The ferrite core helps increase the value without requiring more wire.

The type of inductor below is typically found in passive crossover networks. Again, the core helps increase the inductance without increasing the number of turns. This inductor (referred to as an iron core inductor) is typically considered to be the lowest quality (easiest to drive to its limits) but when properly selected, they work well.