What is the difference between SEO and SEM?
SEO, search engine optimization, is work aimed at being found in the unpaid search results. SEM, search engine marketing, is used loosely: it often means paid search ads, and some sellers use it for paid and unpaid search work together. With paid search ads you usually pay for each click on an ad shown in a labeled ad spot, and Google says no one can buy a better place in its unpaid results. Because the label varies, ask any seller exactly what SEM includes.
Three terms side by side
AI answers are another place customers find businesses; the related answer on GEO vs SEO covers that comparison.
What each costs over time and what happens when you stop paying is in the related answer on whether paid search beats SEO.
| SEO | Paid search | SEM | |
|---|---|---|---|
| What it usually means | Work to be found in the unpaid results | Ads shown in labeled spots on results pages | Often paid search; sometimes paid and unpaid work together |
What Google says
Google's SEO starter guide says SEO is about "helping search engines understand your content, and helping users find your site", and helping them decide whether to visit it. Google Ads Help says that with cost-per-click bidding you pay for each click on your ads, and that this pricing is sometimes known as pay-per-click (PPC). Its page on the Search Network says text ads appear with an "Ad" or "Ads" label on the results page.
On the line between the two, Google's explanation of ads on Search says businesses can advertise in clearly marked ad sections, and that because it only sells ad space, no one can buy better placement in the unpaid results. The related answer on whether Google Ads help your free rankings covers that in full.
What you are buying under each label
As general advice: under SEO you are buying work and time, and any results usually show up as inquiries from search, often over months. Under paid search you usually pay for two things: the ad spend, which goes to the ad platform, and often a fee to whoever manages the ads. Under SEM you may be buying either one or both, so the proposal should say which, in plain words.
The related answer on whether paid search is better than SEO weighs the two as choices for a small business.
Questions to ask a seller who says SEM
- Is this paid ads, SEO or both, and how much of the work is each?
- Is the ad spend included in your fee, or paid separately to the ad platform?
- Will the ads run in an account in my business's name? The related answer on who should own your ad accounts explains why that matters.
- How will I see inquiries from paid and from search separately, each followed to won, lost or still open?
- Is this a monthly arrangement, and what happens to the ads and the account if I stop? The related answer on what a marketing retainer is covers monthly agreements.
Sources
- Google Search Central, "Search Engine Optimization (SEO) Starter Guide" (last updated December 10, 2025)
- Google Ads Help, "Cost-per-click (CPC): Definition"
- Google Ads Help, "About the Google Search Network"
- Google, "Ads on Search - How Google Search Works"
Related answers
- Is paying for Google ads better than SEO for a small business?
- Does paying for Google Ads help you rank higher in the free results?
- What is a marketing retainer, and how should one work?
- GEO vs SEO: what is the difference, and do you need both?
- Who should own your Google Ads and Facebook ad accounts?
SEO for service businesses
Get found on Google by the people already looking for what you sell, in the places you serve, and see which searches turned into inquiries.