What is really sold or bought in the currency market?

Either currencies itself or their derivatives, but you need to be more specific here about currency market with regards to OTC or exchange traded. On the Indian exchanges you can only trade in derivarives.

So if you can specify - it will be better.

Currency market here i mean is exchange traded…

On exchanges in India with trade only in futures and option of currencies.
Futures for 4 currency pairs, i…e. USD/INR + EUR/INR + GBP/INR + JPY/INR
Option trading is only for USD/INR.