The following is an excerpt from a document that I prepared for GMAT aspirants. The entire document can be downloaded here.

Logic is a system of consistent reasoning that allows us to arrive at conclusions using available data and critically check the validity of these conclusions. It can help us with significant choices (e.g. decision making) and can help us analyze data and integrate the relevant parts to decide a future course of action. In general, we can respect the directness of a path even when we don't accept the points at which it begins and ends. Thus, it is possible to distinguish correct reasoning from incorrect reasoning independently of our agreement on substantive matters. Logic is the discipline that studies this distinction by determining the conditions under which the truth of certain beliefs leads to the truth of some other belief. Although logic is undoubtedly a powerful tool for critical thinking, it is but a means to help us arrive at a conclusion and is of little help to examine the validity of the premises which have been used. Logic can therefore give us a consistent and reliable method of inferentially arriving at a conclusion but cannot guarantee the validity of the statements used in constructing an argument.

The chief concern of logic is how the truth of some propositions is connected with the truth of another. Thus, in Critical Reasoning we will usually consider a group of related propositions. An argument is a set of two or more propositions related to each other in such a way that all but one of them (the premises) provide support for the remaining one (the conclusion). The transition or movement from premises to conclusion, the logical connection between them, is the line of reasoning upon which the argument relies. What distinguishes an argument from a mere collection of propositions is the conclusion that can be drawn from the statements in the argument. Thus, for example,

"I had a haircut today.
My computer has crashed yet again.
I need new shoes."

...is just a collection of unrelated propositions; the truth or falsity of each has no bearing on that of the others. But

"Abhay is a doctor.
So Abhay went to medical school, since all doctors have gone to medical school."

...is an argument; the truth of its conclusion, "Abhay went to medical school," is inferentially derived from its premises, "Abhay is a doctor." and "All doctors have gone to medical school." One and the same proposition can (and often does) appear as the conclusion of one line of reasoning but also as one of the premises of another.

In GMAT® critical reasoning, it’s your ability to analyze a given thought stimulus that’s being evaluated, not your opinions on social, political, geographical, scientific religious or any other issues. You have to therefore unconditionally accept the premises* that the Thought Stimulus (TS) contains without debating over the veracity of the statements given.