The author defines and studies a class of fractals called ``post critically finite, self-similar sets'', such as the Sierpiński gasket. These sets possess a sufficient degree of regularity and symmetry so as to allow formation of manageable difference operators such as a discrete Laplacian. The first part expounds the definition and properties of the basic fractals, the generation of certain difference operators and the notion of quasi-harmonic and harmonic functions as kernels of the relevant difference operators. The latter part deals with analogies to classical potential theory in Euclidean domains: the Dirichlet problem for the Poisson equation, the Gauss-Green formula, Dirichlet forms. It should be mentioned that the present approach is quite different from probabilistic methods that have been applied by \textit{Sh. Kusuoka} [Probabilistic methods in mathematical physics, Proc. Taniguchi Int. Symp., Katata and Kyoto/Jap. 1985, 251-274 (1987; Zbl 0645.60081)] and \textit{M. T. Barlow} and \textit{E. A. Perkins} [Probab. Theory Relat. Fields 79, No. 4, 543-623 (1988; Zbl 0635.60090)]. The heavy notations make the paper difficult to read.