I have just been looking at car hire for next christmas and i was given the option of :-

a baby seat
a child seat (1-3)
a child seat (3+)
or a booster seat.

In the US what exactly is a booster seat?

If i can get away with not taking my own i might do so as it was a slight inconveniance last year and even though our luggage was off the plane quick and we were first through immigration, our car seat and buggy didn't come off the plane until all of the other passengers suitcases had which meant we last out of the arrivals hall and into the car hire depot.