What are stereotypes and where do they come from?

On dictionary.com it says a stereotype is, "A conventional, formulaic, and oversimplified conception, opinion, or image." The way I explain what a stereotype is; people's opinions on others about how they look and who they should hang out with. I believe stereotypes are something that has been out since people could speak. I looked for stereotypes on the Internet and I found allot of things about teenage stereotypes and other kinds. I think it's something that is very wrong, to judge people by how they look. But I know I've done it before and probably everyone has done it at some point in their life. I remember on the first day of school how I started stereotyping people in my mind but then when I started to get to know them I found out I was wrong. And I think that happens with a lot of people.
There are stereotypes about the way a person looks, their size, gender, race, religion, even by the music they listen to an a lot more.