A oracle block divided into three parts.
block header,pctfree,pctused.


block header :its contain the information datafiles.


pctfree: its a free space in a datablock for updation and 
insertion of a row.default is 10% of a block size.one more 
thing it's default value is depand of OS also.


pctused : its a space where data are store in a block.its 
default value is 40% of a block size.