what is the difference between data file and data block in oracle
The Logical Storage Structures section in the Oracle Concepts Guide has a good explanation of both data blocks and data files (pages 259 - 296).  There is quite a bit of good information including illustrations, but here are some excerpts:


  data block 
  Smallest logical unit of data storage in Oracle Database.
  Other names for data blocks include Oracle blocks or pages. One data
  block corresponds to a specific number of bytes of physical space on
  disk.
  
  data file
  A physical file on disk that was created by Oracle Database
  and contains the data for a database. The data files can be located
  either in an operating system file system or Oracle ASM disk group.