From Bulbapedia, the community-driven Pokémon encyclopedia.
An entry hazard is any battlefield effect that affects the opponent's Pokémon as they are sent out of their Poké Ball. Any Pokémon already on the battlefield when these effects begin will be unaffected.
Entry hazards are removed by Rapid Spin (user's side) and Defog (Generation VI: both sides; Generation IV-V: only opponent's side).
The following moves create entry hazards of various types.