The Bible doesn't tell us exactly when Jesus will come again -- but it does promise that someday He definitely will return. As Jesus returned to heaven at the end of His ministry, the angels promised His disciples that "This same Jesus... will come back in the same way you have seen him go into heaven" (Acts 1:11).
Most of the "predictions" claiming the world will come to an end in 2012 aren't based on the Bible, but on a calendar from an ancient civilization that actually ended hundreds of years ago. Most experts deny that the calendar even says this -- but if it did, why should we believe it instead of the Bible? Jesus warned, "No one knows about that day or hour, not even the angels in heaven" (Mark 13:32).
Why is Jesus' return important? For one thing, it gives us hope -- hope that someday all the evils of our present world will come to an end, because Christ will rule in glory. It also assures us that God -- not Satan -- is in control, and someday (as the Lord's Prayer says) His Kingdom will come, and His will shall be done on earth as it is in Heaven.
The real question, however, is this: Are you prepared for Christ's coming? Once He comes, it will be too late to repent and turn to Him -- for He comes to judge the living and the dead. Make sure of your salvation by turning to Christ and welcoming Him into your heart and life today.