Enceladus is a moon of Saturn, which has a diameter of about 504 km. It exhibits astonishing tectonic and volcanic activity considering its small size. This study describes physical basis of this activity (energy sources) and internal structure of the satellite with particular emphasis on the subject of liquid water presence. Then the study describes basic tectonic and volcanic processes taking place on Enceladus. The current state of knowledge and some hypotheses which are still subjects of research are presented. In particular, diapirism hypothesis, which accounts for some of the observed features of geological activity on Enceladus, is explained, and simple model of this phenomenon is presented.