A stereotype (from the Greek: στερεός, stereos - solid, τύπος, typos - print) is, like, where you say something that's basically true about someone and get told off for it. Sometimes you project a stereotype on someone which turns out not to be true. Sometimes you find people living up to every single stereotype you ever had about someone, e.g certain rednecks.
Thinking in stereotypes is a normal biological phenomenon designed to decrease the workload of the brain. However, when all flexibility is lost stereotypical thinking can lead to negative social behaviours such as racism and homophobia.