CArd with * on the side. It's usually at the bottom of the sets, in TRR, DS, UF, HP, etc. Usually they're trios, they're basic, and have weird effects if you're losing.
To elaborate more on Pokemon*, they're the EX series of the TCG's representation of Shiny Pokemon. They are always Basic Pokemon and only one are allowed per deck. Like Houndoom implied, check out any set from EX: Unseen Forces to EX: Power Keepers to see the different Pokemon*. ;]
And for the record, Darkness and Metal Energy now come in Basic form, so you can have Metal or Darkness decks now. Houndoom is a good example of a popular card from Darkness decks get built around. Skarmory ex is an example for Metal.