I'm surprised that you didn't try searching first. http://www.quickanddirtytips.com/education/grammar/historic-versus-historical (Don't worry - we native speakers have to check this too.) The difference between "get across" and "put across" is the difference between "get" and "put".

“Historic” is word used to indicate that a specific person, place, or object is of great importance or influence in history. You might say, “On that historic day, the treaty was signed." or "It was a historic occasion, when the Titanic sank.” or "He was the Emperor of an historic dynasty." or "Welcome to London, a city of historic landmarks." "Historical" is used for *anything* which existed (or made to recall history) in the past, whether they were important to history or not. A "historical novel" is a book which is set in the past, but the book is fairly new and has no value to museums or historians. However, if Abraham Lincoln had owned the book or if he used it to accomplish a great feat, the that book would also be would be considered historic. Example: Declaration of Independence - the signing of it was historic, and the document itself is a historical document. However, a replica of the Declaration of Independence is not real and so is not important if it is lost or destroyed, so it is a historical document, but it is not historic. "Put across" and "get across" are different ways of saying the same thing. Both mean "to convey an idea; to communicate; to bridge a gap; to make clear; to convince". Similarly, "get across" can also mean, "to cross a passageway; or to transport something".