I've read a fairly comprehensive introduction guide as to what ETFs are but I am still not sure I get how they are different to say a regular index tracker fund that simply gives you market rates of risk and return.

What am I missing?

The main difference is you get live prices on an ETF so you know what you are going to get when you buy or sell whereas you get an end of day price when you purchase a Unit Trust/OEIC. They are mainly used for index tracker and generally the costs should be slightly lower but you need to look at the small print.