Welcome to LQ!!!
If you would have typed that in google you would got the answer by now :-)
Anyways here is the link: http://en.wikipedia.org/wiki/Device_driver
Here are the answers to your question in simple terms:
What are device drivers ?
Device drivers are softwares which are required by the OS/applicates to communicate with the hardware device.
What purpose do they serve ?
They act as a mediator between the OS/application and the hardware device.
An easy example would be of printer. You do require a driver when you install printer (Modern printers sometimes does not require that as they come inbuilt in the system). When you send print these drivers translate the instruction which device (printer) can understand and get you the desired output.