Every PC comes with a DVD ReWriter drive which is capable of reading DVD disks and writing to blank ones.
You have the option of replacing this drive with a Blue Ray ReWriter drive. This is the only way you can read or watch Blue Ray disks on your PC, it is also a re-writer which means you can save files to higher capacity blank Blue Ray disks.
The Blue Ray drive can also read and write to DVD's, making it a really useful all in one drive unit.