There are many different ways to define the process of critical thinking. The basic idea, though, is that critical thinking is the careful use of reason and logic to examine an issue and to decide upon a course of action.
Critical thinking is careful thinking. It has to examine every step of the thought process. It has to, at every step, be sure that it is not making errors and is not (very importantly) making assumptions without realizing that it is doing so.
Critical thinking is rational and logical. Critical thinking avoids emotional reaction. It avoids thinking in ways that are based on bias or on preconceptions.
Critical thinking is used to make good decisions. It looks at the issues carefully and rationally, examining any assumptions that have been made. It does so to ensure that the decision that is made is the best possible decision given the situation.