Friend Safari Encounters .

In Pokémon X & Y, lies a feature called Friend Safari. It uses the Friend Codes of the Friends on player's 3DS, to allow for some variable encounters.
The encounters in the Friend Safari will have at least two 31 IVs, and also appears to have a higher chance of being shiny.
Additionally, depending on scenario, encountering the Pokémon with Hidden Ability is also possible.

A Friend Safari can have up to 3 encounter slots. The first 2 are guaranteed, however the 3rd slot requires the player's friend to have defeated the Elite 4,
and also be online the same time as the player. (Note: This flag can be activated using PKHeX)

Below is a table of the possible Pokémon found per encounter slot.

Spoiler

Types	Slot 1	Slot 2	Slot 3
Normal
Fire
Fighting
Water
Flying
Grass
Poison
Electric
Ground
Psychic
Rock
Ice
Bug
Dragon
Ghost
Dark
Steel
Fairy