On a voltage-time graph this would appear as a straight horizontal line at a constant voltage.
Car batteries, dry cells and solar cells all provide a direct current (dc) that only flows in one direction.
On a voltage-time graph, this would appear as a curve alternating between positive and negative voltages - the positive and negative values indicate the direction of current flow.
Power stations produce electricity using generators. They generate an alternating current (ac). In the UK, the mains electrical supply is at a frequency of 50 Hertz (Hz) and is delivered to houses at 230 Volts (V).