SEO companies specialize in SEO while Digital Marketing Companies may offer more services like PR, Graphic Design and Social Media Advertising. Look for the company that has a strong focus on the key are you are looking for services. Many marketing agencies that have never offered SEO, or understood it, recently started adding SEO as a service on their websites, to keep up with the trend, and requests from clients, but still find themselves unable to offer professional SEO services due to lack of SEO experience.