A simple stochastic theory for kinetics and energetics of the movement of single motor proteins is presented. The model combines the biochemical cycle of nucleotide hydrolysis with the motor protein translocation. Based on the theory of Markov processes, the model provides the force-velocity relationship, the isometric force, and the stochastic stepping of the motor protein along its one-dimensional track. The theoretical model provides a conceptual framework for realistic studies of motor proteins. Relationship between the present theory and other existing models is discussed.