Term SEO stands for Search Engine Optimization.

Search Engine Optimization is umbrella term for wide variety of techniques and practices that share common goal - acquiring organic traffic from Search Engines.

Depending on where you optimize, there are two types of optimizations:

• On-page
• Off-page

On-page optimizations mean optimizations directly on the site - either technically or semantically, building internal links, making technical optimizations to minimize load time, making sites valid and responsive...

While off-page optimizations are being done in the bigger scope, outside your server/site. Building links, gaining reputation etc.