What is the easy and less complicated definition of predestination [Calvinism]?

I am finding it difficult to understand the Calvinistic ideas of limited atonement and predestination. Does limited atonement mean that only a chosen people called the elect will attain salvation? If yes, will they all be Christians and/or non-Christians as well? How do I know that I am among the elect or not? If everything is predestined, then why preaching the Gospel of our Lord Jesus Christ is necessary? All these ideas just confuses me, please help, but from the Calvinistic Presbyterian point of view and a Biblical basis as well.