CD-R and CD-RW

CD-Rs (CD Recordable) are intended to only be written to once. Once written to, the data cannot be erased, and the disc behaves just like a normal CD-ROM. CD-Rs are more suitable for archiving / long-term storage than CD-RWs because the dye used is less volatile and less prone to deterioration from exposure to light. 


Compact Disc ReWritable (CD-RW) is a rewritable optical disc format. CD-RWs (CD Rewritable) are intended to be used in situations where the data on the disc may need to be replaced. Thus they can be erased and new data added to them. 

Sources: 
http://wiki.answers.com/Q/What_is_the_difference_between_a_CD-R_and_CD-RW
http://en.wikipedia.org/wiki/CD-RW