What does "load" mean and what are the different types?
According to Merriam Webster's dictionary (definition 8b), a "load" is:


  a device to which power is delivered


This may be a circuit, a test device ("dummy" load), or a larger machine or device comprised of many sub-circuits.

The types of electrical loads are typically divided into three categories:


Resistive
Inductive
Capacitive


And of course there are devices which contain a combination of these.


Resistive loads: Resist current flow linearly and cause heat and light (potentially desirable or undesirable). For example, an incandescent light bulb produces light (desirable) but also heat (undesirable). A space heater's element produces heat (desirable) but may also glow. Resistance is measured in ohms for resistive loads, and power is measured in watts.
Inductive loads: Resist changes in current and as such, when you measure the current, it lags (is behind) the voltage. Electromagnetic fields are the key to inductive loads, and as such all motors (fans, pumps, etc), solenoids, and relays are inductive in nature. Inductance is measured in Henrys. The important thing to remember about inductive loads is that they have two types of power, real power and reactive power. The real power is based on the work done by the device (such as what a motor is spinning). The reactive power is that which is drawn from the source to produce magnetic fields. The total power consumed is real and reactive power combined, which is measured in VAR (volts-amps-reactive). It's rather a complicated topic, so check out terms like Power Factor and Real, Reactive and Apparent Power (under AC power).
Capacitive loads: Are for many purposes, the opposite of inductive loads. They resist changes in voltage, and as you'd expect, the voltage lags the current (or more commonly said "current leads voltage"). A capacitor is two conductive surfaces separated by a insulator, which store charge. When power is first applied, current is very high, but drops as the voltage of the charge reaches that of the applied voltage. Capacitance is measured in farads. Like inductive loads, capacitive loads also have reactive power, but it's opposite the polarity of an inductive load. Therefore, a capacitive load has a negative VAR. Capacitive loads are not very common, but things like a flashbulb or a heart defibrillator might be considered a capacitive load (probably a combination load, but you get the idea).