From Disney's Hollywood Studios Wiki
A Cast Member is an employee of a Walt Disney Park. They are easily identified by a white nametag bearing the name of the resort area. It is the job of a Cast Member to better your trip to the park in any way that they can. Most are knowledgeable about their park, and usually the resort area.
Types of Cast Members
Operations - These Cast Member are the most common CMs a Guest will see. These are usually in a shirt with the logo of the attraction at which they work. They are the people responsible for making sure park guest get on and off of an attraction in the safest, and most convenient way possible, so you should try your best to listen to them and follow their instructions.
Merchandise - These Cast Members work at the stores and merchandise stands throughout the parks. They will be the most likely to know about merchandise in the park.
Food and Beverage - These Cast Members work at restaurants and food stands throughout the parks.
Show Keeper - A "Show Keeper" is the name of a custodial Cast Member. You may seem them roaming around the park cleaning various things.
Technicians - These Cast Members are rarely seen by guests. They are responsible to make sure the show or ride is functioning properly, and responsible for the safety of all the performers within the attractions. They usually are wearing something themed to the attraction they work at, so they will blend in easily, to go and fix a situation.
Performer - These Cast Members you will not see unless they are in costume and in character. These cast members are the ones you see in the shows, and will stay in their character until all the guests have left.
Guest Relations - These Cast members work at the Guest Relations area in the parks. If you have a problem, these are the people you want to go see. They are usually wearing Plaid outfits, and most have a Guest Relations button on.
Coordinators - They head over a group of Cast Members within a certain area. They usually wearing solid white business casual style shirts. There is at least one, sometimes two, coordinators per attraction.
Managers - Similar to Coordinators, but one step above on the rank ladder, and usually have a larger area to deal with in a particular day. They are usually in business casual attire. On busy times, their will be one Manager at every attraction, but on lighter times one will go throughout several attractions when needed.
Dream Squad - A "Dream Squad" Cast Member was a very special CM that there was in the parks for the "Year of a Million Dreams" promotion. You never knew when or where you might see them, or what exactly they will be doing at the time, but they were sent to randomly selected areas in the park at randomly selected times, to look for a randomly selected group of people, to give them a randomly selected prize, from special Mouse Ears Hats and Pin Lanyards, to a daily Grand Prize of a stay overnight inside Cinderella's Castle at the Magic Kingdom. Since the promotion is now over, there is no longer a Dream Squad.