Career decision tests come in different forms. You'll find them in books, online or you can make an appointment with a certified career counselor who can administer them.
Some of the simpler ones online are free, and others cost from about $20 up to the price of several hours of a career counsellors time.
Aptitude tests measure your skills in several areas, for example, mechanical reasoning, abstract reasoning, language usage, spatial relations, perceptual ability and so on. The idea is to assess your strengths and weaknesses in specific areas and to check your suitability for certain types of occupations.
Personality tests are obviously an assessment of your personality!
Occupational interest tests are career decision tests designed to identify where your passions are, and what occupations will be most motivating for you. Typically there are lists of careers that you rank in order of your own preference.
These are recommended as the first of the career decision tests that you take so that you first of all find something you really want to be doing. Because if you are motivated and passionate about something, you will quickly learn the skills required.
Career decision tests can be useful to begin to point you in the right direction during a career change. It is often suggested that you do several of them so that you can compare and cross-reference the information. For some people, they bring to light information and ideas that had not been considered previously.
The difficulty, of course, is that people can get 'number-bound', so caught up in the numbers or the huge amount of information that they can't make a decision. Or they keep putting the difficult decision off, because they're not sure what the optimum solution is.
A more significant aspect is that sometimes people don't actually follow their passion, because the tests suggest that they would not be any good at it. They follow their head rather than their heart. They allow others or even computers to decide for them. Which means they never get to live their own lives.
This makes it even more important that you know how to make your own decisions...