Exchange traded funds (ETFs) are a subclass of mutual funds that are very similar to index funds, with the main difference being that you can buy/sell shares whenever the stock market is open as opposed to only once a day. ETFs and index funds are sometimes used interchangeably incorrectly.
An ETF is a security, so it is tradeable and has a share price that rises/falls.
ETFs vs Index Funds
Pick index funds over ETFs if you want to remove the temptation of impulse buying/selling stocks and let you automatically reinvest earnings back into your account.