v$SESS_IO field and number description

Hi,
  could anyone explain to me what does the numbers in v$SESS_IO mean? The Oracle manual is completely silly for this, saying eqations like "Physical reads = number of physical reads".
I need to know what really counts into the "physical reads" and what does the number mean. Is it number of bytes, records, tablespace blocks, disk blocks, filesystem blocks, extents, Fiji shells, ... This information is completely missing.
  And I would be really grateful for description of all the fields.

Thanks
Jan