A conversion is a measurement of something you want the visitor who clicked on the ad to do, an action you want them to take online.
In most cases you want them to buy. Buying is the conversion. The price they paid is the conversion value. This is easiest to understand in an ecommerce business.
When nothing is being sold on the website, then you have to track some other activity.
This could be an application, a request for a quote, scheduling a phone call or appointment or any other kind of form submission. Typically this is done in a Lead Generation setup, where sales happen with follow up or offline.
In lead Generation the Conversion Value is an estimate based on what a lead is worth to the business on average.
If there is absolutely nothing on the website to track, then you don’t really have an online business. (Don’t worry, you can fix that!)
What you want to know is: which of my ad campaigns (and all the way down to the ad group, keyword and ad level) are bringing in the people that convert: the people that buy or become leads?
So you create one conversion measurement (as a Conversion Action in Google Analytics, a Custom Conversion in Facebook, or a Goal in Google Analytics) for each unique action and value that can be taken on your website.
Then the performance of all the campaigns, ad groups, keyword and ads are measured against it.
I’ve seen some people get confused and think that a Conversion Action gets set up for each keyword, or each campaign — the answer to that is No, you don’t setup a conversion per keyword, etc. All the different campaigns, keywords, ads etc are driving traffic towards the destination: the conversion. You want to compare them all against each other. The fewer the conversion actions the better in the beginning.
In Google AdWords when you’re looking at the keywords, ads, ad groups or campaigns, and turn on the Conversions column, the stats / metrics you see there are coming from the Conversions setup … again based on actions the users take after the click.
Click and CTR are what applies most closely to the keyword.
The conversion is what happens after the click.