On certain tracks, beating a certain time in time trial mode will unlock a ghost that can be raced against for an even harder time trial goal. Note that these ghosts will always travel in the same path and thus can be copied and improved upon. The following are the courses that these ghosts are available on, and the time you must beat to first unlock them:
Extra Mode is the name of the mode that is above 150CC. It makes every track played mirrored, so left turns become right and vice versa, and the speed is at 150CC. To unlock Extra Mode, simply beat each grand prix (50, 100, and 150CC).
By beating Extra Mode, all four Grand Prixes being complete, the title screen image changes to a different one permanantly so long as your data is saved.