[kevkrom]

Where does the endzone begin? Inside the line or outside the line?

The entire goal line is part of the end zone. So any part of the ball reaching any part of the line is a TD.

[usmcobra]

I believe you are both wrong, if the playing field is 100 yards long and each goal line is at least six inch long that adds additional foot to the field.

The Endzone begin after the goal line. Therefore you must cross it to score.