Demat A/c and Trasing A/c both are related with secutities market.
A person intended to purchase or shale shares from market and wanted to held in a format other than physical form then they have to open an demat A/c with Depository. By the demat you can take benefit of trading of share in demat format.
Trading A/c is a kind of demat a/c by which demat holder trade securities himself in lieu of trading agent. In today era trading A/c is more famous.