Light source question

What effect does the ambient and specular properties from a light source have on a scene. I understand that in a scene with global ambient light, all surfaces will receive that lighting. And each surface can have a material that specifies if it has any specular highlights. So what does ambient light from a light source do? I''m guessing that all surfaces that are in the range to be affected by the light source will receive the ambient lighting regarldess of which direction they are facing? Same with specular. Will a light with a specular value create highlights on the surfaces it hits? I thought that was the job of the surface''s material.