Effect

Potion effects are conferred by food items called Potions, or alternatively a throwable version called Splash Potions, both of which can be brewed using a Brewing Stand in Beta 1.9 Pre-release 3 in order to acquire the effects.

Behaviour
A living entity(player or mob) affected by a potion effect will be affected in various ways for an amount of time. As of 1.9 pre-release 2, players can open their inventory to see any current potion effects afflicted upon them and their duration. Also, as of 1.9 pre-release 4, players can see swirly particles emanating from their position in first person, and as of 1.9 pre-release 5(6?), this effect can be seen in third person and on players/mobs too.

Parameters
Potion effects are given two parameters on creation: Duration (in ticks) and strength.

History
Potion effects were introduced in Beta 1.8. although no potions were added yet so they were used exclusively for in game actions such as sprinting.

In Beta 1.9 Pre-release 2 actual potions were added and some effects got an alchemical ingredient that correspond to them and Milk was given the ability to cure and clear all potion effects.

In 1.9 pre-releases 2 and 3, Ghast Tears were ingredients in Potions of Healing. Their role was changed to making Potions of Regeneration in 1.9 pre-release 4, and Glistering Melon was introduced for Potions of Healing.