SEO developers are front-end web developers who specialize in optimizing search engine discovery and rankings. They are involved in the technical aspects of the creation, maintenance, and deployment of SEO for websites.
A web developer makes and maintains websites. They are in charge of a site's overall look and feel. Web developers also handle the technical aspects of a website, including its performance (website speed) and capacity (the maximum amount of traffic the site could handle at a given time).