DVD media types are slightly different from CD media types. While CDs come in just two formats - R and RW, DVDs can come in four different formats. These different formats are DVD +, DVD -, DVD R, DVD RW. The different DVD drive types are based on their capabilities with regards to the various DVD media types. DVD+/- RW drives can both read and write from/to +,-, R, and RW DVDs. On the other hand, DVD+RW+R can read and write from/to +R and +RW DVDs. All DVD drives can read CDs and the DVD RW drives can write to CDs.
The speed will determine how fast the data can be read or written on the DVD disc. It is independent of the type of drive.
The drive form factor determines the chassis type that it is compatible with. A DVD Drive for small form factor will not work with a full size or ultra-small form factor chassis. Similarly, it is important that the interface type of the drive be compatible with that of your computer.