A laptop charger has 3 pins - positive,negative,earth at the end which is inserted into the switch board. But, the end inserted into the laptop has only 1 pin(don't know what that is). A battery also positive and negative terminals. So how does this one pin suffice the two pin battery?
That single "pin" is known as a "barrel connector" and has two contacts — one on the outside and a separate one on the inside.
The advantage of this coaxial arrangement is that the connector has no preferred orientation, and can even swivel while in use.