Pass Interference Explained

@ 49Football asks:  at a recent game, the opposing team interfered with our receivers ability to catch the ball.  The official called pass interference, but the team was not given a first down.  Why? 

There are many rule differences between high school (also called Federation Rules), College and the NFL.  There have also been a number of rule changes over the years.  Defensive pass interference is an example.  National Federation Rules for defensive pass interference (DPI) are 15 yards from the previous spot and repeat the down (if not enough for a first down).  As an example, if it was 3rd and 18 on the play where DPI was called and the penalty accepted, in high school the next down would be 3rd and 3 yards to go.  DPI for the NFL and NCAA are both automatic first downs, although it is a spot foul in the NFL while a spot foul up to a maximum 15 yard gain in NCAA.