DVDs are also known as Digital Versatile Disc. This was created in order to replace CDs. CDs are known to store music and some may even be used to store videos but they are not usually adequate in storing videos. Some would require more than 3 discs in order to be completed. DVDs allowed videos to be stored in just one CD. It even has enough space for some deleted scenes, bloopers, and other features.
Xvid is a type of video codec. This would require a big file size in order to show great video quality. Take note that videos that are in Xvid format are not compatible with DVDs but they are usually of better quality as compared to those that you will see in DVDs.