Disney offers Extra Magic Hours for resort guests. They are times when the park is opened early or stays open late and only resort guests are allowed to ride the attractions. During peak times, they can open pretty early like 7 AM for MK and stay open very late especially at MK. They are a nice perk and a good chance to have some less crowded park time. Some people, however, avoid a designated EMH park during regular park hours because they can be more crowded as there will be many resort guests who have selected that park as their choice for the day. One strategy is to use park hoppers to take advantage of EMH and spend the rest of the day in a non-EMH park.
I think that the Disney website, this website, and touringplans.com would have more information including what attractions are open during EMH as not all of them will be available. Hope that helps!