What is SEM (search engine marketing)?

SEM stands for search engine marketing. In most usage it means paid search: ads that appear on search results pages for chosen keywords, bought through Google Ads or Microsoft Advertising. Some people use SEM as an umbrella for both paid search and SEO.

How paid search works

You choose keywords and set a maximum bid per click. On each search, Google calculates an Ad Rank for each eligible ad from the bid, ad and landing page quality, thresholds, competition, the search context and expected impact of ad assets. Ads are shown in order of Ad Rank, and you often pay less than your maximum bid.

Search engine marketing analysis

• Which keywords bring clicks, and which bring sales or leads.
• Cost per click and cost per conversion by keyword and campaign.
• Search terms reports, to add negatives and find new keywords.
• Landing page performance, which also affects Ad Rank.

"Paid SEO" isn't a thing in the sense of buying organic rankings; Google doesn't sell them. The phrase usually means paying someone to do SEO, or it's a mix-up with paid search.

For when to use paid search versus SEO, and how to combine them, see SEO vs SEM.