what is real diffrence between dsb and ssb ?
You are asking about double sideband and single sideband in AM radio transmitters?
You would have to know how amplitude modulation works. I'll assume you do.
If not, read the following:
http://en.wikipedia.org/wiki/Amplitude_modulation
The products of AM are the original carrier and the sum and difference of the carrier and the audio modulation.
So this is a carrier and two sidebands. It is what you listen to as an AM broadcast signal.
DSB removes the carrier but transmits both sidebands.
SSB removes the carrier and also one of the sidebands.
which is better
SSB is better but more expensive to produce. However it is easier to receive.
which is used mostly
SSB is nearly always used now since it gives power advantages (DSB transmits the same information twice) and is easier to receive than DSB.
You might like to read the following article which gives some applications for DSB
http://en.wikipedia.org/wiki/Sideband