For example, people with glasses all read a lot and are intelligent. Or all old ladies knit. While these assumptions might be true for some people with glasses, or some old ladies, BUT they are NOT true for all people who wear glasses, nor all old ladies!
Generally speaking, stereotyping is not considered positive. Categorising/classifying people is both hurtful and harmful. And it is often very unfair - and inaccurate - to take a trait and apply it to everyone in a particular group. After all, not all African-Americans are great basketballers.
Stereotypes are very common in daily life, and in popular culture. Stereotypes can be found in literature for a number of purposes, including comedic effect. However, these days, stereotyping people for comedic effect is generally considered to be in poor taste at best - and is more often considered offensive.