We have already discussed what are bulk deals at our BSE Bulk Deals page. A retail trader is not supposed to disclose whatever he buys or sells on day to day basis to SEBI. But if anyone have to have a deal of significant quantum he needs to disclose the same to the exchange and SEBI. Any such deals happenning in NSE is called NSE Bulk Deals.
As discussed in my site we can use NSE Bulk Deals to spot stocks that can gain in next few sessions. Read THIS POST to know more. You can also track where big investors like Rakesh Jhunjhunwala is investing using bulk deals data.