Early Master Sword (EMS) is a sequence break that uses the N. Faron Twilit Messengers, coupled with a superjump, to get where you aren't supposed to and access the Sacred Grove early. The player can then continue on and obtain the Master Sword long before it was meant to be obtained, and with it, the ability to transform between human and wolf.
Obtaining the Master Sword Early does have its dead ends. The most notable of which is not being able to enter the Goron Mines after completing the Forest Temple and Eldin Twilight. This of course means that you also cannot obtain the bow, nor can the bombs be purchased from Barnes. Because of this, you will have to utilize a new series of tricks in order to skip, not only the Goron Mines, but the use of the bow as well. For additional information on obtaining the bombs, click here. The Goron Mines can be skipped by gaining access to the Lanayru Province early. For more information on how to skip the bow requirements in particular dungeons, visit the appropriate Dungeon Strategies pages.
Because EMS makes it impossible to enter the Goron Mines without the Spinner, it is also an issue that you can't obtain the arrows, and therefore can't use bomb arrows to knock down the stalacmites. Click here for more information.