From Bulbapedia, the community-driven Pokémon encyclopedia.
A Stage 2 Pokémon is a type of evolution card in the Pokémon Trading Card Game. Stage 2 Pokémon evolve from Stage 1 Pokémon.
A Stage 2 Pokémon can be placed in play if the corresponding Stage 1 Pokémon is Active or Benched. Certain effects can circumvent this, such as the Pokémon Breeder Trainer card, that allows a Basic Pokémon to evolve into a Stage 2 Pokémon. All energy and damage on the Stage 1 are put onto the Stage 2, but all Special Conditions are lifted. The Stage 2 Pokémon can only use its Stage 2 attacks, unless the card says otherwise.
Some Pokémon-ex cards are Stage 1 Pokémon if that Pokémon's non-ex card is normally a Stage 1 card. For example, Metagross ex is a Stage 2 card because Metagross is a Stage 2 card.
Pokémon such as Venusaur and Omastar are Stage 2 Pokémon.