An important part of critical thinking is being able to give reasons, whether it is to support or to criticize a certain idea. To be able to do that, one should know how to identify, analyse, and evaluate arguments.
This material is taken from Critical Thinking Web (Open Courseware on critical thinking, logic and creativity)
Critical thinking is the ability to think clearly and rationally about what to do or what to believe. It includes the ability to engage in reflective and independent thinking. Someone with critical thinking skills is able to do the following :
Critical thinking is not a matter of accumulating information. A person with a good memory and who knows a lot of facts is not necessarily good at critical thinking. A critical thinker is able to deduce consequences from what he knows, and he knows how to make use of information to solve problems, and to seek relevant sources of information to inform himself. (Read more at the source.)