Potion

Potions are future craftable food items that confer specific effects in Minecraft. Potion recipes have not yet been defined as of the Beta 1.9 Pre-release. Potion effects were initially introduced in Beta 1.8. If Potions are hacked into the game and their damage value is changed, they will acquire effects. Potions are crafted using water and various ingredients combined in a Brewing Stand. As of Beta 1.9 Pre-release 3, potions can only have one effect. Because the data values for different potions were changed, Potions made in 1.9 Pre-release 2 will be altered if loaded into 1.9 Pre-release 3

There are 4 tiers of potion. (e.g: Speed I, Speed II, Speed III, Speed IV)

History
Alchemy was first hinted at in July 2010, when Notch explained why redstone ore disintegrates into "dust". He states that dust will be used later in potions.

A couple months later, Notch did a Q&A session on Reddit. He was asked about Alchemy, and responded "Oh Yes! Definitely!". All of his posts in that thread are now deleted, but records can be found in other places.

Potion effects were introduced in Beta 1.8. Jeb later tweeted a screenshot showing potion durations and Glass Bottles taken from within Beta 1.9 Pre-release 2.

Notch stated about thinking of putting a lot more potions in a later release

Brewing
For ingredients that go into Potions, and known Potion recipes see Brewing. According to Jeb there are 161 possible potion combinations in Beta 1.9 Pre-release 3, and there may be 2,653 potions in the full Beta 1.9 release.

Types of Potions
Based on information found in minecraft.jar/lang/en_US.lang (of 1.9 pre-release 3), all the current types of potions are listed below:

Names of Potions in 1.9pre3:

Potion of Swiftness, Potion of Slowness, Potion of Haste, Potion of Dullness, Potion of Strength, Potion of Weakness, Potion of Healing, Potion of Harming, Potion of Leaping, Potion of Nausea, Potion of Regeneration, Potion of Resistance, Potion of Fire Resistance, Potion of Water Breathing, Potion of Invisibility, Potion of Blindness, Potion of Night Vision, Potion of Hunger, Potion of Poison

Effects of Potions in 1.9pre3:

Speed, Slowness, Haste, Mining Fatigue, Strength, Weakness, Instant Health, Instant Damage, Jump Boost, Nausea, Regeneration, Resistance, Fire Resistance, Water Breathing, Invisibility, Blindness, Night Vision, Hunger, Poison

Trivia

 * Originally Potions were to be brewed in Cauldrons. Jeb later tweeted that both he and Notch had come up with a different way to brew potions, which was revealed to be Brewing Stands.
 * Taking a potion while already under the effects of another potion does not add onto the effects time, but simply resets it. A potion effect will not "downgrade" if a lower level is activated. For example, if a player has Resistance III, drinking a potion with Resistance II will not have any effect.
 * An Instant Damage III Potion is powerful enough to instantly kill its drinker.
 * Upon death, all potion effects are lost.
 * When you hold a potion, your hotbar turns white.