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 sometimes produce electricity-using magnets. This provides an alternating current (ac). An alternating current is one that changes direction. In the UK ac supply the current changes direction 50 times per second and in the home it is about 230 volts.