What are pre market and post market sessions and orders?
Pre - Open: NSE started the concept of the pre-open session to minimize the volatility of securities during the market open every day. Between 9:00 AM to 9:15 AM is when the pre-market session is conducted on NSE.
During the pre-market session for the first 8 minutes (between 9:00 AM and 9:08 AM) orders are collected, modified or cancelled. You can place limit orders/market orders. After 9.08 AM to 9.15 AM no new orders can be placed, orders placed are matched and trades confirmed. So technically you can place orders only for the first 8 minutes and only on equity segment. More on Pre-market orders on this link.
Post - Open: Similar to pre-market orders, post-market orders are allowed only for equity trading. The post-market session or closing session is open from 3:40 PM to 4:00 PM.
During this session, people can place buy/sell orders in equity (delivery segment using the CNC product code) at the market price but do note that even if you place a market order it will be placed on the exchange at the closing price.
So for example, if the closing price of Reliance at 3:30 PM is Rs. 800, between 3:40 PM and 4:00 PM, you can place market orders to buy/sell Reliance at market price (will be taken at Rs. 800). The post-market session is not very active and you can look at the movement of stocks by opening the Marketwatch window from 3:40 PM to 4:00 PM.