What's G.A? Metric? Dimension?

What is Google Analytics? I can answer this question through how to use G.A.

To say I have a small on-line store to sell handmade rings. I posted advertising with event code. Everyone who uses this code to sign in and make initial purchase would receive VIP card and 10% of the 1st order. After a month, I want to see how many people create their account through this event and get involved in my business. What's the average payment divided by all visitors via this code? This is what Google Analytics can tell. And most gorgeous thing is, it's free!

Then I need to set preferences to run and download a report. How to read the report? What are dimension and metric?

For example, I want to separate the visitors by city name to review the buying habit geographically. So the 'city name' is the dimension and the value of this dimension could be 'Toronto', 'Montreal' and 'Ottawa'.

'Metric' is the value or ratio. For instance, the metric of 'city' could be the population number.