Minecraft Pixelmon servers are based on popular PixelmonMod, which would add Pokemons and a full range of new features to Minecraft. In order to join Pixelmon servers you also need PixelmonMod installed in your client.
Pixelmon servers, Minecraft servers with Pokemons?
On Pixelmon servers you will find many new features, you will be able to find and catch Pokemons, the whole idea behind Pixelmon is to simply bring Pokemons to Minecraft, combining the best of both worlds.
What can you do on Pixelmon servers?
Aside from finding and catching Pokemons, you actually get to play with them. You can play with your Pokemon, you can train it, heal it and even level up your Pixelmon Pokemon.