Personal foul?
 

Last night I was working HL in a JV game. There was a sweep to the other side and just as the play was ending, I saw some trash on the backside...a kid from the defense grabbed an offensive player from behind by the shoulder pads and tried to throw him to the ground. I threw a flag and ran out to tell my WH about it. I told him I had a personal foul and told him what happened. He asked me if it happened after the play and I kind of hesitated, because it was so close to the end of the play I wasn't sure if it happened before the whistle was blown or not. He asked me if I wanted to call it a dead ball foul, so I said yes.

Question #1: What difference does it make if it happened before or after the whistle. I assume it has something to do with penalty enforcement on a live ball foul versus a dead ball foul?

Question #2: Did I correctly report it as a personal foul or should I have called it an unsportsmanlike conduct penalty, or is an USC penalty just a type of personal foul? I realize I may just be worrying about semantics here, but I want to get it right.

If it is deadball, the down counts. On liveball you will replay the down. Also the status of the ball matters (whether it is on a running play, loose ball play, etc.)

Bottom line, when you have a foul, make sure you have the team, number, and status of the ball when you report the foul.

Unsportsmanlike fouls are non-contact fouls. If you have contact, like in your case, it is always a personal foul.

There is no "right" answer, but I think you handled it fine. The action was behind and did not affect the play so I would either not flag it and talk to the player or, if I think a flag was justified (do to the severity of the action plus it may have been visible to all) then I'd probably go with a dead ball foul. The run of however many yards was made without the help of this foul, so I don't want to bring it back unless I have to. The dead ball foul lets A keep the gain they got legally, but still penalizes the cheap shot.

Since there was contact, a PF is the correct call.

I agree. This is what I have been taught by the vets that I have worked with this year. Whenever you can, in this type of situation (rough/cheap play very close to the end of the play and far from the action) try to make it a dead ball foul. Remember, the whistle doesn't necessarily end the play. So if the coach says "I saw that flag on the ground before the whistle, that's a live ball foul!" you can explain that the player going out of bounds, going down, incomplete pass, etc ended the play, not the whistle.

A live ball PF by B will be enforced from the end of the run if it was a running play or if the foul happened after A catches a forward pass. Either way, A gets to keep their gain.

The OP says that the B player "tried" to throw A to the ground. If he failed, I'd talk to him rather than flagging it. If he succeeded, I'd be more likely to flag it, depending on how severe the contact is.

Question 1: before or after the whistle is not the issue; you need to know whether the ball was live or dead. A dead-ball PF is enforced from the succeeding spot; a live ball foul might not be.

Question 2: already answered; contact foul cannot be USC.

Most common fouls?
 

What are the most common Unsportsmanlike fouls? Personal fouls?

Unsportsmanlike fouls - Insulting another players parentage, taunting, foul language, doing an unneeded somersault into the end zone.

Personal fouls - Cheap shots behind the play, face mask, illegal helmet contact, hurdling (Just kidding).

Hurdling is a personal foul.

Unsportsmanlike conduct fouls are an entirely separate entity from personal fouls.

I think the 'just kidding' was in reference to 'most common personal fouls.' Hurdling, while it is a personal foul, is an extremely rare one.

Ah I see that now. Reading is Fundamental (on my part).

Yes, but watching naive referees doing a fake hurdle move instead of the personal foul signal is one you'll never forget.

True: most can't distinguish a fake hurdle from an illegal gallop to save their white hats! :D