Search Engine Optimization (SEO) is the art of doing everything in your power to increase the visibility of your website. This is done by using various techniques including keyword optimization, backlink creation, using accelerated mobile pages (AMP), improving site speed, etc.
So, is there a difference with Search Engine Marketing (SEM)?
Yes, albeit a small one: SEO is a component of the overarching SEM. Both tactics care about bringing more traffic to your website. In a sense, they are pretty much the same, which is not to say the terms are interchangeable.