A prominent purpose of programming languages is to provide instructions to a computer. As such, programming languages differ from most other forms of human expression in that they require a greater degree of precision and completeness. When using a natural language to communicate with other people, human authors and speakers can be ambiguous and make small errors, and still expect their intent to be understood. However, computers do exactly what they are told to do, and cannot understand the code the programmer "intended" to write. The combination of the language definition, the program, and the program's inputs must fully specify the external behavior that occurs when the program is executed.

Programs range from tiny scripts written by individual hobbyists to huge systems written by hundreds of programmers. Programmers range in expertise from novices who need simplicity above all else, to experts who may be comfortable with considerable complexity. Programs must balance speed, size, and simplicity on systems ranging from microcontrollers to supercomputers. Programs may be written once and not change for generations, or they may undergo nearly constant modification. Finally, programmers may simply differ in their tastes: they may be accustomed to discussing problems and expressing them in a particular language.