Theatre and theater, two words pronounced exactly the same way. The E and the R at the end are interchangeable and still the word is correct. Some will tell you this is the British spelling and the American spelling. (Perhaps just to be disrespectful the colonists switched the letters to spite England during the revolution)
Some people will tell you that Theatre is describing the art form in general, the acting, the directing the writing and so forth. While the spelling Theater is describing the building where these performances take place.
While both spellings will work to keep that little red line off the bottom of your word document, there are people who demand you use the "correct" form at the "correct" time. To give my opinion on the subject, if the spelling of theater truly matters to you, then just maybe you need to get a life. There are far more important things to worry about than how someone spells theatre.