In this talk I will introduce the notion of operad and explain how these algebraic structures constitute a powerful tool to treat several (and apparently different) algebraic problems uniformly. I will explain why the theory of operads is closely related to topology and algebraic topology, but also to complex geometry and representation theory. The fun part of the seminar: everything will be treated by using using the language of trees, namely certain directed connected graphs whose combinatorics allows to express the theory of operads in a more visual and intuitive fashion!