SEO is search engine optimization whereas SEM is search engine marketing.
These two techniques will give you similar results, but they work differently.
For example, an HVAC company who uses online marketing would probably see more of a benefit from SEO but not SEM. The reason being is because SEO is more organic and can be a free service if you know how to do it yourself, and have the time. If you want your HVAC company website to rank better on google, but not spend money for google ads. The top few spots are reserved for those who pay for their google ads, but this will guarantee you the top of the organic listings that follow the paid ads. An SEO company will use link building and keyword targeting in order to generate traffic to the site. If more people are clicking on the website, you get more visibility to the HVAC company website on google. Even the website layout has an effect on the visibility. Google ha certain standards for how a website should be set up. SEM is where you are paying for a little extra help. The SEO company can still help you with SEM, but this is where you might spend money on the clicks you generate to guarantee your top stop. Every time a person clicks on your website linke, you have to spend money for it. They’ll always show up at the top, but if no one clicks, you don’t pay. Some companies like having that guaranteed spot. Also, SEM does offer paid marketing tactics on facebook. Facebook is another way to advertise that is becoming one of the leading forms, right behind google.