The ex in the name stands for extra, as indicated by a wide range of attacks that have some sort of additional effect on Pokémon-ex. For example, in the EX Deoxys expansion, there is a Grumpig which possesses an attack called "Extra Ball", which deals additional damage if used against a Pokémon-ex.
The major difference between Pokémon-ex and other types of Pokémon is that when a Pokémon-ex is Knocked Out, two Prize cards are taken, as opposed to the standard one. To compensate for this downside, Pokémon-ex are significantly stronger than other types of Pokémon. In the EX Ruby & Sapphire expansion, Pokémon-ex were basic Pokémon with relatively powerful attacks; however, in following expansions, they had taken on a number of other traits. Pokémon-ex may have up to two Weaknesses or Resistances and a Retreat cost of up to five. Some Pokémon-ex also have a large amount of HP; cards previously had a limit of 120 HP, but Pokémon-ex ignored this rule and had up to 200 HP. Before the Diamond & Pearl Great Encounters expansion, Wailord ex had the title of the most HP: 200. This was later tied with Wailord in the Great Encounters set and Wailord in the Dragons Exalted set, and later surpassed by M Pokémon-EX in the XY series.
Additionally, Pokémon-ex are immune to certain effects: for example, their Poké-Bodies are not deactivated by cards such as Space Center from EX Deoxys, but they are also excluded from the benefits of certain cards, such as Sitrus Berry from EX Unseen Forces, which cannot be attached to them. There are also certain cards that only affect Pokémon-ex, nearly always negatively. The most well known example would be Desert Ruins from EX Hidden Legends, which requires each player to put a damage counter on each of their Pokémon-ex with a maximum HP of 100 or more between turns.
Being a Pokémon-ex affects a Pokémon's name: for example, Blissey cannot evolve from Chansey ex. However, in EX Unseen Forces, Blissey ex had text allowing it to evolve from either Chansey or Chansey ex. This also occurred with Scizor ex. However, Rocket's Scizor ex is only capable of evolving from Rocket's Scyther ex, not Rocket's Scyther.
Many attacks, Poké-Powers and Poké-Bodies on Pokémon-ex are similar or identical in effect to those found on previous cards of the same species, allowing players to continue using popular strategies and tactics while avoiding the need to reprint cards.