A multimeter is usually a hand-held battery powered device capable of several electronic functions. Most can act as a voltmeter and ammeter. Some can measure temperature, resistance, capacitance, conductance and frequency. Older versions may have analogue dials whereas newer versions will have mostly digital LCD displays.
Most multimeters used in schools come with standard 4mm sockets where the probes or test leads connect. These types have obvious advantages if your school uses 4mm leads as standard. They can be used to find faults in circuits, measure the current in a given circuit and take the place of ammeters or voltmeters in most applications.
Most have dials which select the scale on which you would like to work, most will work from several millivolts to several thousand volts, several microamps to several amps meaning that pretty much every application is more than catered for.
The main advantage of the multimeter is that the device can be switched between different functions very easily, eliminating the need for several pieces of equipment. Most compact versions also come with a protective rubber casing, making the unit more robust.
Today, multimeters are relatively cheap to purchase and fairly hard wearing in a school environment.