How can I unload Oracle database tables into a csv files?
Arun,

now that sqlcl is available fromOracle SQL Developer 4.1 EA2 (4.1.0.18.37)
you can use it a lot like the old and famous sqlplus.
sqlcl has an output format setting for csv

set sqlformat csv
spool x.csv
select * from yourtable[s];
spool off


for more info about sqlcl checkout Kris' blog

before sqlcl was around
easiest for this was to use APEX and export the report to csv.
In plain old sqlplus you can do this by using

set lines 9999 -- the appropriate size
set head off  -- no header lines
set colsep ';' --column separator to ;
set pages 0 -- no pages
set feed off
select your data;
spool to_tofile
/
spool off


This works best if the results are to be written on an application server or client. If they have to be written on the database server, utl_file might be a better option.

Ronald.