Quick Definition: The process of using arguments, statements and assumptions to reach a reasonable conclusion
From Rational Wiki:
Logic is the formal study, and use, of the interrelationship between statements in order to determine whether arguments yield useful, coherent and correct results.
A logical argument has a conclusion which follows from its premises. Arguments come in two types, deductive and inductive. In a good inductive argument the truth of the premises renders the conclusion likely, though not certain. Such an argument is described as strong. But further evidence could be added which would weaken an inductive argument so that even if the premises were true the conclusion would no longer be likely.
In a good deductive argument the truth of the premises absolutely guaranties the truth of the conclusion. Such an argument is valid. It is literally impossible for the premises of a valid argument to be true while the conclusion is false. No matter what other facts crop up, the premises imply the conclusion, thus a valid argument is a good deal more powerful than a merely strong one. What you’re really after, though, is a sound argument: a sound argument combines validity with true premises. Since true premises guarantee a true conclusion in a valid argument, and the premises are true, the conclusion of a sound argument must be true as well.