Hot press papers are very smooth, and allow for detailed and vibrant paintings. They are also very popular with coloured pencil artists, as the qualities allow for bright colour applications.
Whereas cold press Watercolor paper has texture to its surface or tooth. They both mean the same thing, little bumps and grooves allow for texture and creative techniques. This paper is sometimes referred to as "Not" which simply mean not hot pressed.
Quality watercolour papers, allow your paints to have more vibrancy.