Voltmeter
Definition
A voltmeter measures voltage — the electrical pressure difference between two points. It connects across a component, in parallel.
Technical definition
An ideal voltmeter has infinite resistance so it draws no current and doesn't disturb the circuit. Real digital meters have ~10 MΩ input resistance, close enough for most circuits.
Example
Across a fresh AA battery a voltmeter reads ~1.5 V; across a dying one it might read 1.1 V — the voltage sag tells you the battery is spent.
Common confusion
A voltmeter goes across (in parallel), never in series. Wiring it in series blocks the circuit's current like a dam; worse, an ammeter wired in parallel shorts the circuit — the two meters are opposites.