Majora's Mask is supposedly the sequal to Ocarina of Time. Though, I've heard that the Zelda game for Dolphin will be the true sequal. This is just a side quest to get you ready for it.
The game plot revolves around a three day time limit in which you have to collect all of the 24(or 25) masks and stop the moon from colliding with the Land of Termina. Termina is the Land where Link is taken when he chases after Stulkid. While there you must find the masks by hel[ping people with their everyday problems. As the days pass people will be doing different things at certain points of the day. It is also your job to keep track of all of those people in you Log Book. That means, finding out what everyone is doing during every second of the day. It seems hard but you get the hang of it quickly.
This game uses a ton of memory so it requires an expansion pak. The reason for this is because of all the people in Termina having to constantly move around all day to complete their daily tasks. Their is also more scenery, enimies, and motion than in Ocarina of Time. At some points ther's even 6 skeletons attacking you at the same time. Also there is a lot more trees and other types of 3-D objects over the land in order to create a more realistic scene.
Since MM uses the same game engine as OoT the controls are very much the same. This is very useful so that if you have already played OoT you don't have to spend a lot of time familiarizing yourself with the controls, and can get into the adventure quickly.
That's about as much as I can think of right now. If I come up with some more I will put it on here.