SEO and SEM are similar in a sense, and much of the confusion about whether they’re the same thing comes from SEM being, in my opinion, rather clumsily named. SEM stands for “search engine marketing” (or sometimes “search engine management”), which is vague and arguably at a high level also what SEO is.
Since the term SEO was around first, it baffles me a bit why anyone thought “search engine marketing” would be a good name for what is basically just paid advertising.
So what’s the difference?
In a nutshell, SEO is the art of manipulating your website and other factors elsewhere on the internet to improve your website’s ability to show up in relevant search results. It’s a bit slower at first to see results, but it also lasts. Get things dialed in and take a month off, you’re still making money.
SEM is paid advertising: Google ads, etc. Results might be immediate; pay money today and make money tomorrow. But the minute you stop putting money in, results disappear.
My guess as to why we don’t simply call it that is because the advertising arena has always been full of clever re-framing of information to guide perception. Take a topic that either has a negative perception or doesn’t sound flashy enough, and fix it with a zingy rename.
It’s the same way that sales reps often call themselves things like “business development manager” to avoid calling themselves what they really are because ‘sales’ has a negative connotation.
It wouldn’t surprise me if the real reason someone conceived of the term “SEM” was really just to put a fancy-sounding label on a service they wanted to offer, and since it sounded related to SEO at a time when all things search engine-related seemed important.
From a pro standpoint it’s fine if someone offers both, or even just management of paid ad campaigns. Both are useful and both have their place in a marketing campaign.
But the term SEM is a little frustrating for some because, in my experience, those who only do paid advertising often know very little about actual SEO, but will masquerade as “SEO experts” and capitalize on the misunderstanding of the terms. (See “Google Certified Experts”.)