Raytracing works in some fashion that light does as it travels trough the world, by casting rays, bouncing off surfaces/passing trough them. Only that rays are cast from the eye/camera and tries to find the intersection between a surface and a light source and returns the result of the path. It's actually closer to a physical simulation than PBR techniques for games has it in its name. And to get a good quality image you need to cast a good amount of rays. Which is computational heavy. Raytracing by itself does not mean PBR though. Next to CPU you also need a lot of Ram for this.
PBR for games as is, is similar to any other traditional rendering technique. Where triangles are clipped and rasterized first and the data between vertices interpolated in each shader stage. After just rasterizing the triangles we already have the fragments to work with. The output pixel is calculated by using some PBR principles by combining Albedo, Metallic, roughness values with additional light settings( which can be precomputed/baked for static objects).The name Physical Based Rendering simply implies that it is using some principles of actual physics model. However it does not mean it actually is, as in it just borrows a few ideas but not the exact model. Many things in game render techniques come down to faking it.
Actually simulating lights bouncing off surfaces is very computationally expensive. And these new raytracting apis for gpus are just one step closer to achieving realistic visuals.
wiki article for ray tracing concept
This one is not directly on game graphics, it's still relevant