Short definition
PPC is any digital advertising model where the advertiser pays only when a user clicks the ad, including Google Ads, Microsoft Ads, Meta, LinkedIn, and TikTok.
In depth
PPC delivers traffic on demand, in contrast to SEO's long compounding curve. The trade-off is per-click cost rises with competition and stops the day budget stops. Sustainable PPC requires three things: a tightly defined target audience, a landing page that matches the ad's promise, and conversion tracking accurate to the dollar. Without those, PPC is a leaky bucket regardless of the platform.
Example
A pool builder pauses Google Display ads (which drove cheap clicks but no leads) and reallocates budget to Search ads with intent keywords; CPL drops 64%.