Adhesion results from the sum of different interactions and (in)compatibilities between two substrates described as adhesive to each other. This review aims to help neophytes to adhesion by answering an apparently simple question: what is adhesion and how does it work ? In order to provide the reader with an overview of the complexity of the subject, adhesion is defined in the most digest possible way, by summarizing the main interactions involved and adhesion theories. The first section of this review defines the different interactions from the smallest scale at which they occur, and along this review to the larger scale at which adhesion phenomenon is observed, using illustrated examples. In a second section the main theories trying to explain adhesion, in a chronological order are reviewed.