Critical thinking involves asking questions rather than memorizing information. Instead of simply collecting the "facts", a critical thinker probes them, looking for underlying assumptions and ideas. Instead of focusing on dates and events in history or symptoms in psychology, you probe for motives, causes an explanation of how these things came to be. A critical thinker cultivates the ability to imagine and value points of view different from his own then strengthens, refines, enlarges, or reshapes his ideas in light of those other perspectives. A critical thinker is both open and skeptical at the same time: receptive to new ideas yet careful to test them against previous expeirence and knowledge.
To put it in one sentence, a critical thinker is an active learner, someone with the ability to shape, not merely absorb, knowledge.