What is Voltage and the volt?

The voltage $U$ between two points of a circuit, in volts ($\mathrm{V}$), is the energy exchanged per unit of charge travelling between them: $1\,\mathrm{V} = 1\,\mathrm{J}/\mathrm{C}$, so a charge $q$ crossing a device under voltage $U$ exchanges the energy $E = qU$ with it. A voltmeter, connected in parallel, measures it.