A computer programmer (often shortened to programmer or coder) figures out the process of designing, writing, testing, debugging/troubleshooting, and maintaining the source code of computer programs. This source code is written in a programming language so the computer can 'understand' it. The code may be a modification of an existing source or something completely new. The purpose of programming is to create a program that produces a certain desired behaviour (customization). The process of writing source code often requires expertise in many different subjects, including knowledge of the application domain, specialized algorithms and formal logic.

The computer programmer also designs a graphical user interface (GUI) so that non-technical users can use the software through easy, point-and-click menu options. The GUI acts as a translator between the user and the software code. Some, especially those working on large projects that involve many programmers, use computer-assisted software engineering (CASE) tools to automate much of the coding process. These tools enable a programmer to concentrate on writing the unique parts of a program. Programmers working on smaller projects often use “programmer environments,” applications that increase productivity by combining compiling, code walk-through, code generation, test data generation, and debugging functions. Programmers also use libraries of basic code that can be modified or customized for a specific application. This approach yields more reliable and consistent programs and increases programmers' productivity by eliminating some routine steps. The programmer will also be responsible for maintaining the program’s health.

As software design has continued to advance, and some programming functions have become automated, programmers have begun to assume some of the responsibilities that were once performed only by software engineers. As a result, some computer programmers now assist software engineers in identifying user needs and designing certain parts of computer programs, as well as other functions.