Prophecy could have two meanings. First, prophecy is the inspired utterance of a prophet of God. Second, the prophecy which we wish to talk about here, is declaring something that is to come to pass, or proclaiming the future.
Part of scripture is prophetical, or is revealing events that will happen at a later time. For instance, For unto us a child is born, unto us a son is given: and the government shall be upon his shoulder: and his name shall be called Wonderful, Counsellor, The mighty God, The everlasting Father, The Prince of Peace. - Isaiah 9:6 This verse was written hundreds of years before Jesus was born. It is prophecy concerning the virgin birth of the Lord Jesus Christ. This particular prophecy was fulfilled in the New Testament.
Some prophecy has, indeed, been fulfilled, as in the verse above. Other prophecy in scripture has not yet at this time been fulfilled. Be certain, though, all things that God says will come to pass will happen.
We can see from prophecy and the fulfillment of it, that God does keep his promises. He never lets us down.