This week, Google published a starter guide for SEO. For the uninitiated into search engine optimisation, it is a good guide to creating websites that fit the basic search algorithms that Google uses. However, it is in no way comprehensive, and omits some pretty important SEM strategies that will help your website rank higher. For instance, links.
However, that aside, if you are looking to understand what your SEO company does for you, and why it works, this is a great place to start.