A glare is a facial expression showing disapproval, fierceness and/or hostility.
Glaring, in some cultures is considered offensive.
A glare may be induced by anger or frustration.
Visually, a glaring person tends to have their eyes fixed and heavily focused on a subject.
This can sometimes be considered synonymous to staring but, in most of the cases, staring is caused due to curiosity and lasts only for a short duration, whereas glaring is caused due to contempt and lasts for a relatively longer duration.