Exchange Traded Funds (ETFs) are a basket of securities you can trade on a stock exchange anytime during the trading day. While stocks represent within individual companies, ETFs offer shares of multiple companies within a packaged bundle, mostly mimicking an index.

Since units of an ETF are solely listed on the stock exchange, they can only be traded on the stock exchange. So, in the case of an index based ETF, when you buy units of an ETF, you are buying underlying shares of various companies  that are a part of the index, replicating the return or performance of that index.

Note: AMOs or After Market are not allowed on ETF.