Every ad campaign is billed one of three ways: per click, per thousand impressions, or per acquisition. Choosing the right one depends on your goal.
If you want traffic to a landing page, CPC keeps you paying only when someone actually clicks through. If you're building brand awareness, CPM gets your ad in front of more eyes for a predictable cost. If you already have conversion tracking in place, CPA lets the platform optimize delivery toward people most likely to convert.
Most advertisers start with CPC, gather data, and graduate to CPA once volume supports it.