In ancient times, scribes who had very important things to record would record them in stone, on cave walls, and sometimes on sheets of wrought or cast metal. When Joseph Smith received the ancient record we now call "The Book of Mormon", it was written on thin sheets of metal that he called "plates". The plates were bound together with metal rings--somewhat in the same way a loose-leaf binder holds together sheets of paper. Here are some descriptions of the plates by various people who saw them, and some who didn't see them directly, but touched them through the cloth in which Joseph wrapped them when hiding them to protect them from the mobs. Another good article on the subject is found here. To the right, see a photo of a replica of what the plates may have looked like based on these various descriptions.
The reason that metal plates were desirable as a means of recording important information for posterity was that they were less prone to decomposition. Gold plates were especially durable because gold doesn't rust or corrode. So important for our day was the message that Mormon was compiling about the rise and fall of his people that he went even to the length of gathering up such a scarce and precious metal, worked it into thin leaves, engraved the record upon them, and hid them in a hill. Later, Joseph would be directed to their hiding place and would be allowed to take possession of and translate them from the ancient language of the Nephite and Lamanite people into English.
The Book of Mormon is a volume of holy scripture comparable to the Bible and is a record of God's dealings with His chosen people in the New World. The main purpose of the Book of Mormon is "to the convincing of Jew and Gentile that JESUS is the CHRIST, the ETERNAL GOD, manifesting himself unto all nations." (Book of Mormon Title Page) It was written by ancient American prophets for our day (Mormon 8:35) and is an American testament of the Lord Jesus Christ.