The shorts answer is: whatever job you want! Mathematics prepares you for a career in a diverse range of areas, and as science and technology become integrated into more and more aspects of our lives, employers are clamoring for employees with mathematical skills and problem-solving abilities.
Some careers that a math major is especially good preparation for are the following:
An important part of many mathematical careers is mathematical modeling. Whether these models are used to describe air flow over the wing of an airplane, traffic movement on highways, or the spread of diseases throughout a population, there are many ways in which a person with a mathematical background can contribute to the process.
Throughout all of this, one might also need to collect and organize data for analysis, and to analyze large amounts of data produced by the model. Different people may be used in different ways throughout the different steps of this process, but in all parts a solid mathematical background and good reasoning skills are crucial, and the tools one learns in a math major are essential.