The first thing that you have to remember is that CDR would mean CD recordable, while CD RW would mean CD rewritable. These two are different from each other in the sense that a CDR can only be used once. This means that once you record something on it, you will not be able to change it anymore. Then, when you use a CD RW, this means that you will be able to record on it multiple times.
For example, you record something on it that you do not like. You can change it again and have something better. Some say that a CD RW is like a hard drive, but the capacity of the CD RW is not as high as that of the hard drive.