As it is studied in a traditional first logic course in college (based on traditional logic), logic is the study of (1) argument form, (2) the qualities (of arguments) of validity, cogency, and soundness, and (3) how to construct, identify, interpret, and evaluate various kinds of arguments. Traditional treatments of logic have included discussion of not just arguments, but the varieties and standards of definitions, as well.
Logic, like mathematics and physics, has a theoretical part and an applied part. Parts (1) and (2) of the above-described definition together describe the theoretical part of logic, and (3) describes the applied part. Just as a nonmathematician learning physics should study mathematics in order to use or apply mathematics well, a nonlogician in any task that requires reasoning, such as confirming rational beliefs, should study logic to learn how to use or apply logic well. Moreover, like mathematics and physics (and many other subjects), one has to practice quite a bit if one wants to gain any facility in using logic. Therefore, logic teachers will frequently assign students to analyze real-life arguments, in roughly the fashion as can be found in the Sherlock Holmes article under the "Holmesian deduction" heading.
See also traditional logic and Aristotelian logic. For comparison, see multi-valued logic.