CPA is cost per action, not acquisition. Action is defined by you.
This bidding mode means you're taking less risk per auction, as you don't pay for impressions or clicks. You pay (sometimes 10x more) to reduce that risk, though, and your reach (how many people see your ad) can be adversely impacted if the Google Brain estimates your click through and conversion rate unfavorably to your auction competitors.
Anyways, action can be whatever you define. It could be a purchase, someone signing up for a mailing list, or someone sending a purchase order, or whatever you can programmatically define.
A click is just opening the landing page from the ad.
