As you saw with the voltage source model, to get a voltage we need two isolated opposite charges.
When two conductors in a circuit are close together, the current flowing tries to get as fast as it can to its destination. Sometimes there's a close enough distance between them that the charges get drawn there instead of continuing along the length of the conductor, until they are pushed out by other charges also trying to get close to their destination.
In that situation, when the charges are just starting to build up, the voltage across that point and the destination point, which from now on we will call ground, starts to build up just as the charges build up.
This delay in the voltage is caused by a property called capacitance. This property is exhibited whenever two conducting or charged layers are separated by an insulating layer, forming what is called a capacitor.
The capacitance depends on various factors like the thickness of the conducting material (thicker means it can hold more charge), the surface area of the conducting layers (more area means more charges have a close spot to the opposite layer) and the insulating ability of the separating material (called dielectric).
