In order to charge your laptop's battery, you require direct current or DC. However, all plug points will supply alternating current or AC instead.
The function of the laptop adapter is to convert this readily available AC to DC in order to charge your laptop's battery.
The internal components of this adapter (or 'brick') consist of several power electronics including rectifiers, regulators and filters and also a transformer, in order to carry out AC-DC conversion. This transformer is fairly heavy and accounts for most of the weight of the adapter.
As a result of this design, you can use the same laptop adapter in any country (regardless of the type of AC used) and still charge your laptop without any issues, since the adapter SHOULD convert an AC signal of any voltage and frequency (within reason) to output the required DC.
I myself have been using a laptop purchased in the US (120 V, 60 Hz) in India (230 V, 50 Hz) for 3 years, with no problems, requiring only a pin adapter to allow the US plug to fit into an Indian plug point.
