A software tester looks for software errors
Software testing is closely connected to software engineering: software testers are professionals in testing software and in discovering errors that the developer of the code being tested did not plan or foresee. Software testers are in charge of increasing confidence in the software and detecting errors. The background: time and time again, software does not work as it should. Software testers are responsible for finding these errors in advance and increasing user satisfaction. These errors can be caused by a number of issues.
Software testers work on all levels of software development: the smallest units of the code are checked, as well as the finished software as a whole. Software testers do not only look for errors in the code that cause it to fail, but also errors caused by misunderstandings between the developers and the users.
Software testers prevent expensive software errors
A professional software tester does not just accept the developer: they put themselves into the user’s shoes. This is where the software tester’s creative thought process begins. They try to anticipate user inputs that are wrong. One example would be when entering the password. Generally, you will need to pay attention to uppercase and lowercase letters because it is case-sensitive. Usually you are made aware of the fact that the password is incorrect. Nowadays, a further piece of information is given: make sure that the caps lock is not in use. If it is pressed, a small light is displayed. Without this note, the user could be completely confused. Maybe they mistyped it. They try it again several times and suddenly receive a new message: your password has been blocked, please contact your system administrator.
This scenario still happens nowadays in every software, in a different form. The input of the user violates a business rule that user knows nothing about. Good programs give an appropriate error and help message. Software testers detect these errors and return their finds to the developer company, which will then optimise their program.
You can find more information on software testers and software testing on Wikipedia: https://en.wikipedia.org/wiki/Software_testing
And on the German-language Encyclopedia of Business Informatics: http://www.enzyklopaedie-der-wirtschaftsinformatik.de/lexikon/is-management/Systementwicklung/Hauptaktivitaten-der-Systementwicklung/Software-Implementierung/Testen-von-Software/