What is a blur tool in Photoshop?

Exactly what it says it does : blur. It blurs the image layer that you have selected. This can be helpful if you are trying to combine multiple layers and you want them to blend better, if you want to simulate depth of field in a photograph (selectively blurring),if you want to censor parts of an image or remove an annoying photo-bomber, or if your image (or parts of it) is just to sharp for your taste.