Here is a breakdown of the Mantoux or PPD test:

You are given a very small needle with a liquid in it in your arm. You must then wait somewhere in the neighborhood of 48 hours (but I think 72 hours is the absolute longest you can wait) and return to have the results read by a doctor or nurse.

A negative result means that everything is definitely OK and you are cleared of TB.

A positive result does NOT necessarily mean you have TB or have been exposed to it. It just indicates that you MAY have had exposure at some point. If you receive a positive result, typically a chest x-ray is done to rule out TB. The doctor may also ask you if you have had any symptoms of active TB just to make sure. Once you have a positive result, you will always have a positive result and next time you need to be tested, you have to request a chest X-ray rather than taking the PPD. I am one of those people that just has a reaction to the test and has to be x-rayed every time an employer requires a TB test. It is a real pain sometimes.

If you have previously been treated for a positive TB infection that is another story and you should definitely talk to your doctor about that.