A normal compact disk has a size of 700 MB but it only holds data upto 650 MB. What is the other 50 MB utilized for?
Not all CD's hold the same amount of data. There were a couple different versions produced that attempted to squeeze a few more megabytes out of the standard. The 650Mb/74min and 700Mb/80min standards are the most common. Check out the table in the wiki within the capacity section.
They actually hold a little more information beyond the stated capacity of each size and this extra data space is used for error correction data.
1Kb = 1024 bytes
1Mb = 1024x1024Kb=1048576Bytes = 1Mb
650Mb = 1024x1024x650 = 681574400 bytes
I think the rest is CRC checksums.
I think this Wikipedia entry explains it quite well: http://en.wikipedia.org/wiki/Compact_Disc