insert /*+ append */ into tab2 select * from tab2 ???

Allow the row to be loaded using DIRECT PATH (by-passing the SGA). Works since
Oracle 8 I think. You use this when you want to load large amounts of data fast.

Some docs on the subject:

http://technet.oracle.com/docs/products/oracle8i/doc_library/817_doc/server.817/a76965/c21dlins.htm#365

It means add the data to table tab2 without creating any redo-logs , thus the insertion of data is faster as the step involved in creating the redo logs doesnt come into picture.

I want to know what is the + means here?
and what is the  /*...*/ means??

how they can be used in select statement? please give example for update /delete/modify :)

/*..... */  is used for commenting....
/*+......*/ is used for providing hints to oracle on how to execute the commands....