Critical thinking involves uses logical and rational thought processes to explain a particular issue. It involves an objective examination of facts, and ways these facts come together to explain an issue.
Important to the understanding of critical thinking is the ability to think more deeply and profoundly about whatever topic or theory it is that you are focusing on and then to be able to analyse its various strengths and weaknesses. It is important in critical thinking to always be aware of the limits of knowledge.
Your own question implies one trait valable in much critical thinking: the ability to compare and contrast. The more one compares and contrasts two things, the more one is likely to learn about the individuality and even the uniqueness of each thing being compared and contrasted. The ability to see resemblances and perceive differences seems crucial to much critical thinking.
There are lots of ways to define critical thinking. All of them have to do with being logical and rational and with looking for things that are not obvious.
Two such definitions are that critical thinking is A) thinking that focuses on deciding what we should do or what we should believe about certain situations or B) the process of analyzing our own thinking to see if we are thinking in the correct ways. In both cases, we see that we are meant to use logic to look below the surface and get to deeper understandings of situations.
The only difference here is that the first emphasizes only our focus on the issue at hand where the second focuses on critiquing our own though process.