Mycology is the study of fungi (fungus). This includes their genetics, their biochemical properties (the chemical processes in them), their classification, their use to humans and their dangers (poisonous or infectious) . Fungi are a source of tinder (easily combustible material to light fires), medicines, food, and entheogens (a psychoactive substance). A biologist who studies mycology is called a mycologist.
Mycology is closely related to phytopathology (the study of plant diseases) because most plant diseases are caused by fungi. Historically, mycology was a branch of botany (plant biology). Now fungi are more closely related to animals than to plants.
Fungi are fundamental (essential) for life on earth: they're symbionts (an organism living in a mutually beneficial relationship with another organism from a different species), they have potency in breaking sown complex organic biomolecules and they play a role in our global carbon cycle.