Copywriting is the ability, specialty, or capacity of writing correct content and composing proper material in general. Copywriters are content writers. SEO copywriting is the ability to create content having the SEO aspect in mind. That means making sure you keep the keywords in mind and you use the important words with the correct density.
It’s an agency specialized in writing SEO-optimized copies.
It depends on the rest of your partners you have. You can work with an SEO agency, an SEO copywriting agency, a PPC agency, a Social Media Agency, a PR agency, and a development agency. The issue is that Marketing doesn’t run in silos. All activities should be in sync and coordinated. The optimal would be to utilize one agency for all activities that will help you to have a single point of reference and responsibility for the final result.