This paper aims at revealing the essence of volunteer computing from the point of view of formal abstraction. At first, three elements of volunteer computing are analyzed and outlined. It included some concepts of component and computing resource in volunteer computing. One formal model and method concerning volunteers are introduced. Resource roles such as the basic elements of systems are identified in the abstract model, and modeling volunteer systems by basic elements. Relationships among elements and relationship changes are also characterized, based on set theory and operational reduction rules. And take XtremWeb as an example. These elements and relationships are specified in a formal manner. All of the results can help to build a firm foundation for research of formalisms for volunteer computing.