Hi,
EMH stands for Extra Magic Hours. If you're staying at a Disney onsite hotel, on selected dates throughout the 15th anniversary year (so up to end of March 08 at least) you either get morning or evening EMH. If its morning, you get in the park 2 hours before the official park opening time, and some rides are up and running for you. If its evening EMH, you get to stay in the park 2 hours after the official closing time, more rides open, and only people entitled to EMH get to ride.
Elaine