Room service is included in your cruise fare, as is all the food served in the buffet and main dining room, plus other places depending on the ship. In other words you don't have to pay a penny extra to eat. But if you want an upscale dining experience with dry-aged steaks, then you have to pay extra for it such as at Royal Caribbean's Chops restaurant featured on most of their ships. $30 per person including tip, but not including wine or other bar drinks. Same holds true for the other lines.
Now, unless you are on a line such as Seabourn or Regent you are not going to get lobster and caviar delivered to your cabin. As I recall from my Celebrity cruises in the 1990s, the room service menu was typical of most cruise lines - sandwiches, salads, cake, cookies. Those in suites could get (and still can, by the way) breakfast, lunch, and dinner served ensuite during those meal hours, from the dining room menus. This is also true with Royal Caribbean.
On Celebrity's Solstice-class ships their are cabin classes that come with dinner seating in the intimate Restaurant Blu, which does serve some great food and it is included in your cruise fare. Most consider Blu to be better than the main dining rooms.