first you should know the meaning of a SEO agency --

Using search engine optimization an SEO agency may assist businesses and organizations in improving their online presence and increasing traffic to their website. Typical services provided by a reputable SEO firm include website audits keyword research on-page optimization content production link building and analytics.

There are mainly 4 types of SEO services and those are:

On-page SEO
Off-page SEO
Technical SEO
Local SEO