A digital multimeter can be used to measure voltages, currents, and resistances. Some multimeters can measure more values. It is the most used measurement tool when working with electronics.
Principle: An Analog to Digital converter translates the analogue value at the inputs of the meter to a digital value on the screen. In the early days, multimeters were still analogue, bulky and less easy to read-off.
Caution: for measuring current, a different, fused input socket is used. Make sure you check if the input is in the right socket! A short circuit may be the result.
The current measuring input typically has a resistance of several milliOhms, where the voltage measuring input typically has a resistance of 10 MegaOhms. In the resistance measure setting, the multimeter outputs a small current (and thus voltage) itself.
We made a tutorial on using multimeters.