Ghast

The Ghast is a floating jellyfish-esque mob that shoots fireballs from its mouth. Its body is a 4 by 4 by 4 block cube with 9 tentacle-like body parts emerging from the bottom. They first appeared in the Halloween Update. Ghasts are able to spawn anywhere in the Nether, as long as there is sufficient space. While an extremely rare occurrence, ghasts may spawn near portals in the regular world if given enough space to do so.

Ghasts normally float around with their eyes and mouth closed, making a sound like cooing children. They open their eyes and mouth when attacking. They make a chirp-like noise when shooting and scream loudly when taking damage. The sounds that ghasts make are currently not affected by distance, so they always sound very close, regardless of their actual position.

Netherrack, glass and dirt blocks can be destroyed by the Ghast's fireballs. Blocks originally ignited once struck, adding an extra hazard to fighting Ghasts, though as of 1.0.2 this is no longer the case. Based on the materials that are fireball-proof, the explosion strength of the Ghast's fireballs is likely between 15 and 24. Materials with an explosion resistance above that range and therefore cannot be destroyed include:
 * Cobblestone
 * Stone
 * Brick
 * Diamond
 * Gold
 * Iron
 * Iron doors
 * Obsidian.

If a Ghast's fireball hits a portal (or close to it) the portal will be closed, requiring re-ignition. If the player manages to get a Ghast to fire at the portal while it is closed, it can be reignited using the fireball.

Ghasts will not fire at the player without line-of-sight, and this is blocked by glass, leaves or portals making safe scenic paths through the Nether a viable possibility. When not moving, Ghasts within firing range will face the player, giving the impression that they can see through transparent materials. Ghasts can draw line-of-sight through curtains of lava, and their fireballs can pass through it meaning players near a pillar of lava can be struck without seeing the source.

Ghasts drop Sulphur when killed, but this can be hard to collect - as it is the only mob that floats, the sulphur may fall into lava or an inaccessible area.

Fighting Ghasts
Ghasts attack with projectile fireballs. The player can reflect these fireballs by hitting them with anything before they impact, even bare hands, arrows or fishing rods. It is possible to kill Ghasts by reflecting their attacks back at them. Although rather tricky to pull off, you can make this easier by shooting an arrow, fishing rod, or snowball at the fireball, then looking directly at the Ghast, or where it will be in the next second or two, and it will send the fireball in that direction once hit. This way, if a Ghast is out of reach from your projectiles, you can "aim" its own fireballs back at it, instead of having to run directly at the ball to fire it in the right direction. The Ghast hitbox includes the tentacles, so aiming for the area where the tentacles meet the body increases the chance of a hit.

Ghasts may be hurt with normal weapons if the player can get close enough or has a bow. It is possible to use a fishing rod to draw Ghasts in near enough to hit with a sword. A close ghast is also easier to hit with a bow, though the drawback of this method is less time to dodge fireballs. When aiming your bow be sure to lead your shots as Ghasts can quickly change trajectory and move quite fast. Depending on how far away the Ghast is it may be necessary to aim above the Ghast as well to gain extra range.

Ghasts may also be set on fire, but fire and lava don't actually damage them. A similar effect is seen with zombie pigmen.

It is disputed that Ghasts can be killed by lava or fire if exposed to it for a long enough time. There have been reports of zombie pigmen dying while in lava, which raises doubt that ghasts and zombie pigmen are completely lava/fireproof.

Much like using neutral or enemy mobs as shields against arrows fired by skeletons in the regular world, it is feasible to use zombie pigmen as shields against Ghasts in the Nether. It is possible to injure or kill zombie pigmen in this fashion without ever making them hostile to the player.

Drops
Beta:
 * 0-2 units of Sulphur

Trivia

 * Ghasts can be seen on the map when using certain map editors.
 * Ghasts can "cancel" their fireballs if the targeted player has an obstruction between the Ghast and himself.