There are seven Hidden Skills that can be acquired during your travels. While the first one is automatically found early in the game, the remaining six can only be learned after activating their respective Howling Stones. After howling into them, a Golden Wolf will appear somewhere in Hyrule - his position will be marked on your map. You'll learn a new technique upon meeting with him.. It should be noted that the order that the Hidden Skills are learned is linear, which means it does not matter what order the Howling Stones are triggered in.

Hidden Skills

1. Ending Blow (learned automatically prior to Forest Temple)
2. Shield Attack
3. Back Slice
4. Helm Splitter
5. Mortal Draw
6. Jump Strike
7. Great Spin

Howling Stone Locations

* Death Mountain - In the area with multiple geysers.
* North Zora's River - Northeast of the boat shop.
* North Faron Woods - After crossing the gaps (with Midna's help), you'll find it just prior to the Sacred Grotto's entrance.
* Lake Hylia - Climb the ladder on the southern cliff section to reach it.
* Snowpeak - You'll find it as you climb the mountain. It's south of the secret cave's buried entrance.
* Hidden Village - Break through the ground floor windows of the western building.

Golden Wolf Locations (in order of appearance)

1. Ordona Province - In the Spring where you first met Epona and Llia.
2. Castle Town - On a pillar east of Castle Town's entrance.
3. Castle Town - In the field south of Castle Town. He'll be around the staircase on the left side.
4. Gerudo Desert - Northeast corner.
5. Kakariko Village - Near the rear of the graveyard.
6. North Castle Town - Near the entrance to Hyrule Castle

... really hoped this helped! ^_~