From Bulbapedia, the community-driven Pokémon encyclopedia.
The Unlimited format of the Pokémon Trading Card Game allows any card that was once playable in Play! Pokémon tournaments (that is, all cards except Worlds edition decks, jumbo cards, Ancient Mew, Birthday Pikachu, and otherwise unplayable cards) can be used. It is not as popular or as commonly used as the Standard format or Expanded format and is not used in any tournaments sanctioned by Play! Pokémon organized play. It can, however, be used in Pokémon Leagues if the League Leader wishes to allow it.
In the Pokémon Trading Card Game Online's Versus Mode and Tournament Mode, any deck composed of cards currently available in the game are allowed in this play type (including Standard format decks and Theme decks). Currently, only HeartGold & SoulSilver, Black & White, and XY sets and their expansion are available.
Because of the lack of restrictions in Unlimited, decks can use combinations of cards which are not legal in Standard format or Expanded format play. Cards such as Porygon2 and Seeker which never appeared in the same Standard format, are frequently used together. Despite the large variety of cards available, only a few strategies make up what is generally considered the highest "tier" in Unlimited play, with most competitive decks being either "donk" decks focused on winning by KOing your opponent's only Pokémon in play, or "lock" decks focused on preventing the opponent from taking meaningful actions. The formers is more difficult for the player going first due to the current first turn rules.
Deck archetype contains a more complete list of well-known deck designs similar to the two examples listed above.