UX is about how people interact with a product. Visual design is about what they see while interacting.
So UX will guide you towards what features are important and ways of exposing those features so the end user has the best experience. UX also tends to focus on testing assumptions.
Visual design will guide you towards making sure the elements on the page are aestheticly pleasing and that they communicate what is trying to communicated. Google is famous for testing it's shade of blue, but, IMO, great design can't be tested, but rather comes from a designer understanding her audience and the story she is trying to tell.
So UX will guide you towards what features are important and ways of exposing those features so the end user has the best experience. UX also tends to focus on testing assumptions.
Visual design will guide you towards making sure the elements on the page are aestheticly pleasing and that they communicate what is trying to communicated. Google is famous for testing it's shade of blue, but, IMO, great design can't be tested, but rather comes from a designer understanding her audience and the story she is trying to tell.