Index fund v ETF

[justakiwi]

I am confused about the difference between an index fund, and an ETF. Can someone please explain the difference? I have googled, read many articles and watched various podcasts including some by Jack Bogle, but I still can’t get my head around it.

For example, I hold Smartshares USF500 - according to Sharesies:

“US 500 fund invests in the Vanguard S&P 500 ETF which is an index fund.”

So what is the difference between that and say, AMP Global Shares Index Fund?