This document describes the Multi-Order Coverage map method (MOC) to specify arbitrary sky regions. The goal is to be able to provide a very fast comparison mechanism between coverage maps. The mechanism is based on the HEALPix sky tessellation algorithm. It is essentially a simple way to map regions of the sky into hierarchically grouped predefined cells. The encoding method described in this document allows one to define and manipulate any region of the sky in such a way that basic operations like union, intersection, equality test can be performed very efficiently. It is dedicated to Virtual Observatory (VO) applications or VO data servers for building efficient procedures for which mapping knowledge is required like generic catalog cross-match, computation of data set intersections, or similar operations.