It is in the form of a small animal with a star shaped "hat" upon its primarily white body.
According to the game's story line it is a mythical Pokémon that grants wishes upon waking up from a deep sleep. The wishes are be written down on purple papered wish tags (which are an actual item in the Pokémon games but currently unusable as of date) and placed upon Jirachi's star points. When the creature awakens its only goals are to grant the wishes written upon the tags. Upon fulfilling them, it returns to sleep.
Currently, Jirachi is only attainable at a Pokémon Center In New York, Osaka, Tokyo and an additional center in Japan. A special machine transfers a randomly selected pokémon with a 1 in 1000 chance of reciving Jirachi. Recently the machines use has been restricted to special events most recently done to the New York PKMN Center.
As it is exceedingly rare in the Pokémon world / fan community, very little is known about it. It is set to "star" in the upcoming 5th (6th?) Pokémon Movie.
This pokémon is of the Psychic type as is common with its predecessors Celebi and Mew also mythical Pokémon with unique stories.
It has been said that Jirachi can be obtained by using a GameShark unit.