What is Hidden Power, and how does it work? How do I get the Hidden Power I want? Why do I see people using these HP [type] moves? Hidden Power is a move that claims to be Normal, but is actually anything but that. Its type (any of the 16 types that are NOT Normal) and base power (31-70) are both determined by the DVs of the Pokémon using them. Its type will determine whether the move uses physical or Special-based Attack/Defense stats to deal damage. No matter what type your HP is, though, it can be countered by Counter, but Mirror Coat will fail.
The only control you have over what Hidden Power you get is if you catch or breed multiple Pokémon until you get one that has the desired Hidden Power. It's up to you whether you think that work is going to be worth it for a move that may or may not be helpful.
If you have a good type/strength of Hidden Power and the stats to back it up, Hidden Power can be an excellent move. HP Water/Ice/Grass is fairly common on Electric-type Pokémon, who can use it to fight back against the Ground-type Pokémon they usually can't do much damage to. But something like HP Psychic on a Snorlax just isn't going to cut it thanks to his stats.
Another explanation of the move: http://www.angelfire.com/pa4/jman/hidden_power.html Handy chart of what DVs yield what HP: http://database.stumpnet.net/hpchart.html
Not much people are smart enough like me - Chaosmare