the goal is to explain what a stack is and why people think they are interesting objects to study. i'll start by saying what people (who want to classify a certain class of mathematical objects, say triangles or elliptic curves or...) expect from a moduli space and why it can easily happen that such a moduli `space' doesn't exist. this is the situation in which stacks get their chance. they are some kind of a generalization of a the notion of a `space', where the meaning of space depends on what you are interested in (for example, in the context of algebraic geometry one might say that stacks are a generalization of the notion of a scheme). i'll then use an example to motivate the axioms appearing in the definition of a stack. depending on how much time i have i will say what an algebraic stack is and maybe how deligne and mumford employed the notion to prove something interesting.