Effect

Potion effects are conferred by food items called Potions, which can currently be hacked into the game in order to acquire the effects.

History
Potion effects were introduced in Beta 1.8. There are currently 19 defined "potion effects" in Minecraft. Two are given by Player actions, Two more by food items, One by a Cave Spider's attack, Six others work but aren't used yet, and Eight have no effect. Jeb later tweeted a potential potion effects screenshot taken from within from Beta 1.9 Pre-release 2. Jeb has also tweeted about the Blindness potion effect, confirming its upcoming existence.

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