Eevee had three evolutionary forms in the first-generation Pokémon games: Vaporeon, Jolteon, and Flareon. Each evolution is associated with a different elemental type and is triggered by using a different evolution stone.
A Water Stone evolves Eevee into Vaporeon, a Thunder Stone produces Jolteon, and a Fire Stone produces Flareon. This branching evolution made Eevee unusual in Pokémon Red and Blue, where most species followed a single fixed evolutionary path.
Eevee later gained five additional evolutions: Espeon, Umbreon, Leafeon, Glaceon, and Sylveon. That brought the total to eight forms in the modern games. Because the later forms are much more familiar to newer players, questions about Eevee must specify whether they mean the original generation or the full current set of Eeveelutions.