Effect Monsters are orange Monster Cards. Effect Monsters have at least one Card Effect or Condition.
Effect Monsters are designated by the text "Effect" on the Type/Ability line. Originally, Effect Monsters, which also had abilities (such as Toon and Gemini) or were Tuner Monsters, would display their subtype and not "Effect". From Battle Pack 3: Monster League onward, all Effect Monsters now include "Effect" regardless of their abilities or status as a Tuner Monster. Regardless, Monsters that have abilities are always Effect Monsters (although this is not the case for Tuner Monsters).
Extra Deck Monsters
Fusion, Ritual, Synchro, Xyz, Pendulum, and Link Monsters can also be Effect Monsters.
Some Effect Monsters have powerful Effects, but low ATK and DEF for their Level. Additionally, some Cards with higher ATK or DEF for their Level have negative Effects (such as "Giant Orc").