Once you learn the Song of Healing from the Happy Mask Salesman you will be able to undo the Skull Kid’s spell that has you trapped in the form of a Deku Scrub. In doing so you will also gain the Deku Mask, which allows you to take the form of the Deku Scrub whenever you please.
As a Deku, Link will be able to use the launch flowers scattered around Termina to fly short distances, and attack by either spinning or spitting bubbles. Deku Link is quite vulnerable to fire attacks and falling from great heights, so it would be wise to avoid both while wearing this mask.

Follow the Ghost of Darmani to the Goron alter by using the Lens of Truth and play the Song of Healing to ease his suffering. In return you will gain the Goron Mask that will allow you to take the form of a Goron.
As Goron Link you will be table to travel quite quickly by curling into a ball and rolling around, as well as attack your enemies with mighty fire punches. Like Deku Link, Goron Link is quite weak when it comes to heights, as well as a complete mess when it comes to water.

Mikau the Zora can be found on the shores of the Great Bay, seriously injured. Play the Song of Healing for him and you’ll get the Zora Mask, allowing Link to take the form of a Zora.
Zora Link is obviously a swift and powerful swimmer, as well as able to walk along the bottom of the sea. Zora Link can also create an electric barrier that surrounds him, damaging any enemies that come into contact with it. He can also use his Zora fins like boomerangs for ranged attacks. You might, however, want to be wary of Fire and Ice attacks when in Zora form.

Return the lost fairy in Clock Town to the Great Fairy Shrine in North Clock Town to return the Great Fairy to her normal form. As a reward you’ll receive this mask. It will make collecting the Stray Fairies in the dungeons far easier, as they naturally come towards this mask.

You’ll notice a Goron in the Mountain Village area wearing this mask. To get it from him first head into the Goron City. Light all the torches in there, then destroy all the barrels spinning around the chandelier. You will be rewarded with the Sirloin Rock for your troubles. Bring this to the Goron wearing the mask and he’ll give it to in exchange for the rock. This mask allows you speak to the frogs located throughout Termina.

Enter the tunnel to the left of the Deku Palace after completing the Woodfall Temple. You’ll meet the Deku Butler there, who will guide you through the maze-like caves (to make it easier try wearing the Bunny Hood). Make it to the end and get this mask, which allows you to smell mushrooms when they are close.

On the first or second night head to the Laundry Pool area where you will find a man playing a crank organ. Speak to him and listen to his woes, and you’ll get the Bremen Mask. Wear it and animals will believe you to be their leader and follow you around as you march.

Head to the Chicken Shack at Romani Ranch and speak with the man you find there. Put on the Bremen Mask and collect all the small chickens, marching around with them until they grow to full size. Complete this task and the Bunny Hood is yours. This is probably one of the most useful masks, as it increases Link’s speed significantly.

Finish both Romani and Cremia’s Notebook tasks (outlined in the Bombers’ Notebook section) to get this mask. Wearing it will get you access to the Milk Bar in Clock Town, the most exclusive of exclusive clubs.

To get this mask complete the first Gold Skulltula dungeon. As for its function, anyone who has played Ocarina of Time will be familiar with it. Wear this mask when speaking to the one-eyed statues and you’ll get some interesting information. In addition, you’ll also be able to hear the thoughts of animals when you speak to them.

Stop the thief from robbing the Old Lady in North Clock Town at midnight on the first night and you’ll be rewarded with the Blast Mask. An excellent substitute if you happen to be short on bombs, it does have the serious drawback of costing Link quite a few hearts. If you happen to have your shield raised while you detonate it, however, you’ll manage to avoid taking damage.

To get this mask, head outside of Clock Town and go to the mushroom shaped rocks. At the stroke of midnight a dancing man will appear. Play him the Song of Healing and he’ll give you this mask, which will allow Link to perform a mystical and ancient dance.

Beat the Gorman Brothers in the horse race and you’ll win this mask. Once you get this mask you’ll be able to enter Ikana Canyon as well as confront the Garo Warriors.

Defeat the Giant Skeleton in the Ikana Graveyard to get this mask. With it you’ll be able to talk to the undead enemies you encounter.

To get this mask, head to the small village right before the Stone Tower in Ikana Canyon. Head into the house shaped like a music box and into the basement of that house. You’ll find a deformed man/mummy down there. Play the Song of Healing and you will receive the mask. This makes it so Gibdos and ReDeads ignore you.

This useful mask can be gained by first going to the entrance to Ikana Canyon. You’ll see a circle of stones. Use the Lens of Truth and you’ll see an invisible guard. Give him a Red Potion and he’ll give you this mask in return. This mask makes you blend into the background, making you invisible to many enemies.

Perform the Notebook task for Toto the Zora and the Gorman Circus Leader and you’ll get this mask. If you wear it while doing Cremia’s Notebook assignment you won’t have to fend off the milk thieves, making it a much easier job.

If you stopped Sakon the Thief from robbing the old woman on the first night, this mask will be available for purchase at the Curiosity Shop on the third day. Using it allows you to stay awake during the stories Granny will tell you at the Stock Pot Inn.

This mask is earned by defeating the Eyegore mini-boss in the Stone Tower Temple. Wearing it, as the name would imply, turns Link into a giant (at the cost of magic). Unfortunately, it can only be used when fighting boss of this dungeon, Twinmold.

Simply talk to Madame Aroma, the fat woman in the Mayor’s House, and she’ll give you this mask. It’s use is pretty much exclusive to the Anju & Kafei Notebook task outlined in that section.

You gain this mask in the process of completing the Anju & Kafei Notebook assignment. When wearing this mask and standing in a circle of plants, attacking the plants will cause the mysterious Keaton to emerge, who will then quiz you with questions.

Also earned through completing the Anju & Kafei sidequest, this hat allows Link to look into the various mailboxes throughout Clock Town. There’s all sorts of interesting stuff you can find in them.

Again, gained by completing the Anju & Kafei missions. While pretty, it doesn’t serve much of a purpose. You may want to try speaking to the Mayor while wearing it, though.

Wearing this mask transforms Link into the Fierce Deity (or “Oni-Link” in Japanese; roughly “Evil Link”). While undeniably cool, the Fierce Deity Mask can only be worn during boss battles. Getting it requires Link to have collected all the other masks in the game, then completing the mini-dungeon on the Moon before facing Majora.