The Poison status's only effect is that it causes constant HP loss at a rate of 12.5% (1/8) maximum HP at the end of every turn in battle. When not in battle a Poisoned Pokémon will keep this status and lose 1 HP for every 4 steps taken. In fourth generation games when a Poisoned Pokémon reaches 1 HP, outside of battle, the status is removed.
Poison is one of the most used status effects in a competitive environment.
There are several abilities that will prevent the Poison status, or remove it. Abilities that will prevent this status altogether include: Immunity and Leaf Guard when the weather is Strong Sunlight. Natural Cure will remove this status effect when switching out of battle. Hydration will remove the status, after being inflicted, when the weather is Heavy Rain.