What is tomato blight?

Tomato blight is a disease caused by an oomycete (a fungus-like organism) called Phytophthora infestans. This same disease also affects potatoes, having caused the infamous potato blight famines in Ireland in the 1800s. 

Tomato blight may be confused with blossom end rot, which is actually caused by a nutrient deficiency rather than a pathogen/disease.