Just to expand on Fitzer's post:
The main battery is a great big thing, almost the same width as your laptop. It powers the laptop so that you can use it without it being connected to thie mains. It is designed for easy removal/insertion as some people like to carry a spare with them.
The BIOS battery is a coin cell, hidden within the laptop and is used to supply power to keep the date/time correct etc when the unit is powered off. They usually last the life of the computer or thereabouts.