Alternatively referred to as a circuit analyzer, multitester, or VOM (Volt-Ohm-Meter), a multimeter is a device used to test and measure current, voltage, and resistance. In the picture is an example of a digital multimeter. The multimeter connects to a circuit using two (red and black/positive and negative) probes.
Before testing any electrical component with a multimeter, make sure to properly ground the system and yourself. Leave the device plugged into an electrical wall socket if the outlet is grounded.