How do capacitors work and what are its units?

Capacitors are an electrical component made up of two plates with an insulator between them.

Electrons flow onto one of these plates as current flows around the circuit. This gives one of the plates a negative charge. 

As electrons build up on this plate, they repel electrons positioned on the plate opposite (negative-negative repulsion). 

This gives the other plate a positive charge. For each electron flowing onto the first plate, one is repelled on the plate opposite. This gives the plates an equal and opposite charge.

NO current flows through the capacitor, charge simply accumulates on each of the plates. This accumulation of charge is effectively stored energy. Electrons will continue to flow onto the negative plate until such a time as the negative plate repels electrons. This means the potential difference across the plates is equal to the emf of the power supply. 

Each capacitor varies with how many charges its plates can hold, and thus how much potential difference can exist across the plates. This ratio of charge stored to the maximum potential difference between plates is defined as the capacitance. It has units F (the farad, after Faraday). One farad is equal to one coulomb per volt, so one farad is a very large quanitity! Usually capacitors have values of the order millifarad or microfarad.