Are you serious about your lawn? If so, this page from Cornell University could be an indispensible reference for all the diseases that can plague your turf. Each link on the page provides an overview of the disease, symptoms, pictures, disease cycle information and control strategies. After reading, you will be able to differentiate gray snow mold from a necrotic ring spot and be well on your way to being a lawn care expert.
Credit: Turf Diseases Factsheets
