About Eevee
Eevee is a small, brown, long-eared, furry creature, from the Pokémon video game, trading card, manga and anime series. They have generally well-rounded statistics and abilites for an un-evolved Pokémon.
It is a Normal-type pokemon and belongs to the Ground egg group. It is the only "Evolution species". Eevee was introduced in the very first generation of the video games, with the Pokedex description, "Its genetic code is irregular. It may mutate if it is exposed to radiation from element stones." Eevee also makes various appearances in the animated series, especially since two major characters (May/Haruka and Gary/Shigeru) both have an Eevee - the former evolved hers into a Glaceon, while the latter evolved his into an Umbreon.
One of its special abilities is Run Away, which allows Eevee a greater chance to run away from battles with wild Pokémon more easily. The other one is appropiately named Adaptability, which raises the same-type attack bonus to 2.0.
Evolving Eevee
When it comes to evolution, Eevee has the most unique and varied range of evolved forms. A trainer may wish to transform Eevee into a Vaporeon (water type) with a Water Stone, Jolteon (electric) with a Thunder Stone, or Flareon (fire) with a Fire Stone.
There are two other evolutions which do not require magical stones (however, in Pokémon XD, Sun Shard and Moon Shard can be used); when an Eevee is perfectly happy with its trainer, it will become either an Espeon (psychic) during the daytime, or Umbreon (dark) if it is nighttime.
Pokémon Pearl and Pokémon Diamond introduce two new evolutions: Leafia (grass) and Glacia (ice). Eevee evolves into Leafia when you are training near Moss Rock or Glacia if you are near Ice Rock.
Alternate Colours
In the Gold and Silver versions of Pokémon, Eevee may also appear in an alternative or "shiny" colour, which is silvery grey. (Read more about Alternate Colouration at Bulbapedia.)