Multimeter

Measure voltage, current, resistance and continuity — and avoid blowing the fuse.

Beginner

Voltage — in parallel with the component. Use DC (V⎓) or AC (V~) mode; "True RMS" meters read non-sinusoidal AC correctly.
Current — in series: break the circuit and move the red lead to the A or mA socket. Measuring voltage with leads in the current socket is a short circuit — the fuse blows (or worse).
Resistance / continuity — only with the circuit unpowered.
Diode test — shows the forward voltage (~0.6 V silicon, ~2 V red LED).

Check the CAT rating (CAT II for sockets, CAT III for distribution panels) before measuring mains.

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