Vaporeon
Eevee can evolve into a Vaporeon at any level with the use of a Water Stone. Vaporeon, a Water-type Pokemon, does double the damage against Fire-, Ground- and Rock-type Pokemon. However, both Electric- and Grass-type do double damage against Vaporeon. Vaporeon's strength is in caves, where the majority of Pokemon are Ground- and Rock-type.
Flareon
When a Fire Stone is used, Eevee evolves into Flareon, a Fire-type Pokemon. Flareon is most effective against Grass-, Ice- and Bug-type Pokemon, while weak to Ground-, Rock- and Water-type Pokemon. Flareon is most effective in forests, when the majority of Pokemon are of the Grass- and Bug-type, and should not be used in caves when it can be avoided.
Jolteon
When a Thunder Stone is used, Eevee evolves into Jolteon. An Electric-type Pokemon, Jolteon is strong against Flying- and Water-type Pokemon, making him ideal to use when in or around the water. However, not only do Ground-type moves hurt Jolteon more than other moves, Electric-type moves do not work on Ground-type Pokemon at all.
Espeon
When Eevee gains a level during the daytime when his happiness as at its maximum, he evolves into Espeon, a Psychic-type Pokemon. Espeon is strong against Bug- and Fight-type Pokemon, though his high attack power and powerful Psychic moves make him a good choice against most Pokemon. However, he is weak to Bug-type Pokemon and he has lower health points than the other evolutions.
Umbreon
When Eevee gains a level at night with maximum happiness, he evolves into Umbreon, a Dark-type Pokemon. Umbreon is strong against Ghost- and Psychic-type Pokemon and weak against Bug- and Fight-type Pokemon. While Umbreon has a strong Attack statistic, this is offset by his low Defense statistic.
Leafeon
When Eevee gains a level near the moss-covered rock in Eterna Forest, he evolves into Leafeon, one of the two new evolutions in "Pokemon Pearl." Leafeon, a Grass-type Pokemon, is strong against Ground-, Rock- and Water-type Pokemon. However, Leafeon has the most weaknesses of any Eevee evolution, being weak to Bug-, Fire-, Flying-, Ice- and Poison-type Pokemon.
Glaceon
When Eevee gains a level near the ice-covered rock on Route 217, he evolves into Glaceon, the second new evolution in "Pokemon Pearl." An Ice-type, Glaceon is strong against Dragon-, Flying-, Grass- and Ground-type Pokemon while weak against Fight-, Fire-, Ground- and Steel-type Pokemon. Glaceon works well against Dragon-type Pokemon due to them also being Flying-type, making him ideal for a battle against a trainer who relies on those kinds of Pokemon.