Search engine advertising (SEA) is the practice of paying to place ads directly in search results, usually through auction-based platforms like Google Ads or Microsoft Advertising. Advertisers bid on keywords and typically pay only when someone clicks, so their listing appears above or beside the unpaid organic results.
Position isn't sold to the highest bidder alone. Each time a query triggers your keyword, the engine calculates Ad Rank — roughly your maximum bid multiplied by Quality Score, a measure of expected click-through, ad relevance, and landing-page experience. A well-targeted ad with a strong Quality Score can outrank a competitor bidding more, and you're charged just enough to hold your slot rather than your full bid.
Paid search buys instant visibility; the moment the budget stops, so does the traffic. Organic SEO compounds over time but takes months to mature. Picture an emergency plumber bidding on "burst pipe Toronto" at an $18 cost-per-click — because one job is worth $600, the maths works. For "how to stop a dripping tap," a low-value informational query, ranking a helpful article organically is far cheaper than paying per click. Most mature programs run both, letting paid cover urgent, high-intent terms while organic earns the informational long tail.
SEA is the paid half of search engine marketing; the unpaid half is SEO.