There are three basic types of search engine optimization services, and anyone spending money on SEO should be familiar with these differences.
Understanding them helps you know how to hire the right kind of SEO talent, be it agency, freelance or employee. It also helps you more accurately define their true level of experience, and identify what kind of conversion and ROI to expect from the dollars you spend.
It also might help you avoid joining the discouragingly high percent of companies dissatisfied with their SEO, bouncing from one provider to another.