Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...f5/4. Nc3/4...fxe4/5. Nxe4/5...d5/6. Nxe5/6...dxe4/7. Nxc6/7...Qg5/8. Qe2/8...Nf6/9. f4/9...Qxf4
White now has the simple plan of establishing a knight on e5 guarded by a pawn on d4 (which isn't going to get taken en passant because it'll go there with a discovered attack on the Black queen). The two possible ways of carrying out this plan of Ne5 and d4 are (you guessed it):
The immediate 10.Ne5+ means that the Black queen will definitely go to h4 after 11.d4, whereas 10.d4 gives it the option of retreating to d6 instead. If she declines that option and goes to h4 anyway, the two lines merge into one.
White still has the Nxa7+ option. But 10. Nxa7 could have been played a move ago if that's what White wanted. It's not terribly consistent with sacrificing the pawn on f4.