Hi Joseph, well the simple answer would be that affinity photo is a program with tons of tools to edit and modify photos. So if you need to change photo colours, have filters, remove objects from photos or blend photos together this is the app for you. Check it out here.
Affinity designer is more for logo design. Making posters, brochures, business cards and wireframes for websites. This is basically the app you would need. Check it out here.
Both work hand in hand together.