Program classification based on kinds of an environment interface, program specification, language kernel, etc. is analyzed. The classification is intended to be a scientific basis for creating proper development methods for each program class. The following three program classes were defined: programs implementing functions (program-functions), reactive systems, and programming language processors. Definitions of program classes are based on the basic notions of a program, a program specification, requirements, etc. which were previously systematically defined. The paper may be used for teaching university students specialized in software engineering. The program-function and reactive systems classes include more than 90 % of all programs. There are also the following two meta-level classes: programming language processors and operating systems which are considerably more complex. The four mentioned program classes are not the full collection of program classes. Other program classes exist, for example, programs connected with data bases.