Logic is the science of reasoning and relationships between ideas. An expert in logic is a logician. Spock, a Star Trek character, is the ultimate logician — when asked why not, he answers, "Because it isn't logical."
A logician examines statements to decide whether they are true and is active in the fields of math and computer science. A very simple example of what a logician does can be given with the word or. What exactly does it mean if you say "A or B?" A logician would say that it can mean: 1. A but not B, 2. B but not A, or 3. A and B (this sounds weird but in logic this is true). Or, however, does not mean "not A and not B." Logicians usually deal with much more complex ideas and statements than this.