What is the difference between clicks and impression?

you mean cpc and cpm?
cpc cost per click : $/click
and cpm cost per impression: $/1000 impression

Your Ads visible in front the user that is called Impression
When user came your websites via click your ads that's called Clicks

Impression is measured how many times your ads shown on a search result page.
Clicks is measured how many times user visit to your website through ads

When your ad loads and displays in front of a user, that is one impression. Well, you can't have clicks without impressions. Users need to see your ads before they can decide whether or not to click them. This is typically referred to as CPM, or Cost Per Mille, where Mille means thousand.