Of the many masterpieces and relics of note within the basilica, the most awe inspiring has to be the Scala Sancta (Holy Stairs), the marble staircase leading to the praetorium of Pilate in Jerusalem. Moved from Jerusalem to Rome by St. Helen, these steps were sanctified by the footsteps of Jesus during His Passion. The marble stairs are accessed through openings in the wooden risers which are built around it to protect the precious relic from the thousands of pilgrims who come annually.
For the complete history of the basilica, you can go here. And, following is a short video documentary of St. John Lateran: