A multimeter is quite probably the most useful tool an electrician can utilise. These devices are basically pieces of testing equipment that allow for the measurement of more than one parameter by the use of the same device, and this means that users can choose the exact test procedure they want and use a multimeter to gain a measurement of that reading.
The capability of a multimeter depends entirely on the design and construction. Basic multimeters will generally be able to only measures such things as voltages or resistance across small ranges, but higher end multimeters might be able to measure those same parameters across higher ranges, and also test things such as capacitance or temperature levels.
Multimeters are generally broken down into two distinct categories. The first of these - analogue - are devices that utlise electromechanical components to display a measured value. This basically means that the analogue multimeter displays results as a needle moving across a scale, or set of scales. They're generally best suited to detecting sudden fluctuations in measurement.
The other category is the digital multimeter, or DMM. This device uses special electrical components to display measured values and it displays these results as a digital reading, usually on a built-in LCD screen. They are generally regarded as being more accurate than analogue devices and are used much more commonly in the modern testing industry.
These devices also vary greatly in size and appearance. It's quite usual for most electricians to be spotted out and about with a handheld multimeter that's portable enough to not be cumbersome, but there's also bench multimeters that are generally used in laboratory and calibration procedures that are capable of generating much more accurate results.