As stated above, SEO is comprised of two different activities.
On-Page SEO includes:
Incorporating selective keyword naturally into title tags, meta descriptions, heading tags, alt text, etc.
Blog posts and page copy that is written and optimized with quality
Clean and formatting page URLS
Optimized page load speed
Google authorship incorporated
Social sharing integration within your content
And much more!
Off-Page SEO includes:
Creating a high quality, natural backlink profile (aka having other high quality/authoritative sites link to your site naturally)
Social sharing signals
Social bookmarking (Stumbleupon, Reddit)
List goes on here too!
“Search engine marketing is a form of Internet marketing that involved the promotion of websites by increasing their visibility in search engine results pages (SERPS) through optimization and advertising.” SEM includes SEO tactics, as well as several other search marketing tactics.