Charge can flow in a material under the influence of an external electric field. Eventually the internal field due to the repositioned charge cancels the external electric field resulting in zero current flow. To maintain a potential drop (and flow of charge) requires an external energy source, ie. EMF (battery, power supply, signal generator, etc.). We will deal with two types of EMFs:
The ideal voltage source is able to maintain a constant voltage
regardless of the current it must put out ( 
is possible).
The ideal current source is able to maintain a constant current
regardless of the voltage needed ( 
is possible).
Because a battery cannot produce an infinite amount of current, a model for the behavior of a battery is to put an internal resistance in series with an ideal voltage source (zero resistance). Real-life EMFs can always be approximated with ideal EMFs and appropriate combinations of other circuit elements.