For guests staying on Walt Disney World property, Extra Magic Hours (or EMH as they are referred to by Disney-geeks like me) are a nice little for perk for staying on property. The parks have EMHs on certain days and they can be morning or evening hours.
Currently, morning Extra Magic Hours allows resort guests entrance into the designated park 1 hour early before general admission. At the gate, guests are required to show their Key to World card along with their park ticket (if purchased separately). The evening Extra Magic Hours allows guests of the resort to stay in the designated park 3 hours after park closing for the public. At this time, Keys to the World cards are required to be shown before entering an attraction during EMH.
Beginning in January 2013, Evening EMHs will be changing to reflect only an additional 2 hours in the theme parks vs the current 2 hours. There is no change to the Morning EMHs.