what's acomputer programmer do?

The diversity of roles that a programmer can fill is quite extensive. Basically your average run-of-the-mil programmer is an individual who designs, writes, and tests software. What particular platform the software is developed for can range from childrens’ toys, PCs and game consoles to intelligent industrial machinery. The programmer basically enters a series of instructions which are compiled into binary information that a processing unit can follow.

The majority of current day programming projects require entire teams of programmers working in specialised roles. Take game development for example. Your average game requires programmers to fill specialised roles including AI modelling, 3D graphics, sound processing, 2D graphics, physics modelling, interface development and multi-user support. Each role requires study in its related disciplines. Then there are the project/team leaders who are usually less involved with the actual programming then they are with the efficient allocation of tasks and resources.

One could really write an entire essay on the diversity of programming roles and positions.

I would say that a programmer does not even design or test software.  It depends very much on the size and complexity of the project.  For example the people who wrote Windows 2000 obviously had little input into the design and could not have done any testing of the final thing.