Capacitors

Could anyone explain this?
-Whilst capacitor is charging, work is done removing charge from one plate and depositing it to the other.

-can anyone please summarise briefly what we need to know on capacitors?

Assuming you are referring to A-level and not degree level?

Try here. Good place to start. http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capcon.html#c1

You will also need to know about: dielectrics, permittivity, charge distribution, electric fields, rudimentary use in AC circuits, phase relationship between current and voltage and applications of capacitors as energy storage devices and transducers and finally some real world applications such as AC to DC power supply conversion, accelerometers, microphones, touch sensors and AC applications such as signal conditioning filters and power factor correction for industrial use.