Here is a copy/paste from an earlier submission of mine regarding the polygraph. I didn’t change any of the formatting, so you’ll see that my answer was originally meant for someone else. However, these answers are still relevant to your question.

I submitted a paper in a college about whether the polygraph should be used in investigations at all for a forensic criminalistics course. There's a rather long history with respect to the device. You probably know the courts have ruled that, in general, the results from a polygraph are not admissible in court. This dates back to the outcome of a 1923 D.C. Circuit of Appeals Court case known as Frye v. United States where it was debated whether the polygraph met the standard of general acceptance (accepted within the "relevant scientific community").

I have no formal experience with the polygraph. This was just an assignment for a class. The device itself measures physiological responses that are often "associated" with deception. These responses include your heart rate, your breathing patterns, any sweating, systolic blood pressure, and sometimes a voice-stress analyzer.

You asked if polygraph tests are reliable. In my opinion, they are "reliable" to the degree of the relationship between the examiner administering the questions and the person undergoing the test; it's more of a game of psychological dominance on the part of the examiner. This goes to your second question on whether it is easy to "fake" it. It's not so much about whether it's easy to fake, but rather your perception of the device as well as your reaction to the questions being asked. This moves into your third question. Polygraph tests are used to measure your responses between two categories of questions: control questions and relevant questions. There are neutral questions as well that serve to create a baseline response such as "Is your first name ____?"

Control questions are questions where pretty much none of us could answer with 100% honesty such as "Have you ever told a lie to your parents?" or "Did you ever cheat on a test in school?". The examiner knows this and will assume you're not telling the truth. This is done by design. These questions aren't the ones that matter.

Next, you'll be asked a relevant question in which the examiner wants to get VERY specific on a certain subject such as with the use of illegal drugs or whether you knowingly omitted anything on your background questionnaire. Your responses to these questions are measured against your responses to your control questions. This is what helps to determine your "truthfulness".

That's pretty much how they work.