Ghosts. The undead of the Pokémon world. There are only four which fit in this group, yet they're the most complicated of all. Their states. The ghosts have the ability to switch between four 'forms', each having their own advantages and disadvantages.

Solid

What all other Pokémon classify as. Their bodies are solid, and can be harmed and damaged by physical blows.

Non-solid or Ethereal

The 'ghostly' state, the ghost must concentrate to a minimal degree to remain in this form. They are visible, yet all attempts at physical damage pass right through. In this form, although immune to physical, they are now much more susceptible to 'special' attacks than the solid form, and take more (about +50%) damage from them. Fire, Electricity, non-physical Grass, energy beams, etcetera. Water does not classify as such, despite the game's 'special' classification of it, and will pass through in this state. It also has the ability to pass through walls, roofs, floors, and any obstacle in the arena in this form, so long as they keep moving and are not in solid space for more than five seconds. This means Ghost Pokémon can NOT go and hide underground in the ethereal form, though they can go under momentarily, coming back out a few seconds later for perhaps a surprise attack.

Gaseous

Not quite ethereal, a little closer to solid. Like ethereal, it can't be damaged by physical attacks, but they still produce a little effect, a 'funny feeling' that can become quite distracting if it's a large or fast 'hit'. 'Special' attacks are still more effective than solid, but not quite as bad as ethereal (about +33%). Gaseous ghosts are not capable of passing through walls, but can fit through even the tiniest of gaps, say, a crack or split in a wall. The time it takes to pass through this hole though will depend on it's size, obviously a while for a tiny crack or pinhole.

Invisible

The ghost becomes invisible, and cannot be seen. Although harder to hit because it can't be targeted, it will take damage from most any hit, as it is in its solid state whilst invisible. The invisible form is limited to 2 rounds per ghost per battle.

As well, attacks are restricted in some states. Invisible, the ghosts can only attack with physical or mind-affecting psychic moves. In ethereal or gaseous, no physical attacks can be used.

Unless the ghost is ordered to be in a certain state, it remains fluctuating between solid and ethereal/gaseous. If it sees a solid/physical attack coming, it will easily become ethereal or gaseous to avoid the damage. If a 'special' attack is coming at it, it will shift to solid form to minimize the damage, as it is more susceptible to it in ethereal form.

If a ghost is caught by surprise by a physical attack, say, from behind, each of the four ghosts have different possibilities of hitting, which are explained in their descriptions. Surprise special attack chances are the inverse of mentioned physical surprise chances. (e.g. Haunter: 40% chance of physical surprise hit --> 60% of more effective special hit.) The time in ethereal/gaseous is split evenly among the two, so surprise special hits that hit with a damage bonus is half the time +50%, and half the time +33%.

Gastly

The only ghost that is in ethereal/gaseous form all the time. It can not attain a solid form except when it becomes invisible. As a trade-off for its near complete physical immunity, it remains the slowest of all four ghosts.

Misdreavus

Aside from when it desires to be in solid form, it is almost always gaseous (about 85% of the time). Surprise physical attacks will almost always pass through Misdreavus. The lowest to the ground of the three floating ghosts when at rest and not 'flying', so when it does lapse into solid form, it can take its share of scratches on rough terrain, especially when moving fast.

Haunter

The middle of the 3 possibly solid ghosts. When not expecting them, physical attacks will most often miss, yet they do have a solid chance of hitting (about 40%).

Gengar

The most solid ghost, and the only one to actually stand on the ground, if this fastest ghost is not flying. His solid form is more dominant at rest (near 66%), and he will more often than not be hit by surprise physical attacks.

Shedinja

Shedinja is be solid at all times and will always be hit by surprise special and physical attacks since it is a shell of Nincada when it evolves. However, due to its Wonder Guard ability, Shedinja can only feel those attacks that are super effective against it. For more information on Shedinja, check the Creating a Squad section.

Shuppet and Banette

These Pokemon are mostly in solid form about 85% of the time (the rest of the time, the Pokemon will appear gaseous). These Pokemon always float above ground as high as their trainer is tall. Surprise physical attacks will connect with Shuppet and Banette most of the time (about 75%). These Pokemon, like Misdreavus, cannot pass through solid objects.

Sableye

Sableye is in solid form all the time and travels only by walking or running along the ground. Surprise physical attacks will connect with Sableye about 50% of the time. This Pokemon cannot pass through solid objects.

Duskull

Because of its sneaking nature, Duskull will be considered about 40% solid and visible when it is stationary. When it moves, its form becomes ethereal and harder to see and can pass through any object. This makes Duskull appear to teleport from one place to another. Duskull moves slightly faster than Gastly. Duskull is more likely to be hit by surprise physical attacks while he is stationary (about 50%).

Dusclops

Also somewhat ethereal, Dusclops is considered to be more solid standing on the ground (about 60%) than floating in the air (about 50%). This Pokemon can float a few inches off the ground if ordered to do so. Dusclops is a very slow floater/mover because of its size and is susceptible to be hit by all special attacks. Surprise physical attacks are more likely to connect with Dusclops while it is on the ground (80%) than it is while he is floating (70%).

Additional Information

All ghosts are immune to mentally affecting Psychic moves (sig or not, unless specified it can affect ghost [usually Dark type]) in all forms.

Telekinetic-based Psychic moves (Psychic, Disable) have NO effect on a ghost in the ethereal or invisible state.

Ghosts in the gaseous state are only affected by Disable, and it only reduces its ease of movement by 1/4, or lowering their speed to 3/4 of its original as long as it is held.
Ghosts in the solid state, telekinetic moves are actually somewhat effective. Disable is 3/4 effective, holding its target mostly in place, only allowing minimal movement, 1/4 speed, at great effort from the ghost. Psychic is 1/2 effective, and although the ghost can be moved, in most cases the ghost can counter that force with attempted movement of its own in the opposite direction.

Once a telekinetic move has locked it's effects on a ghost, the ghost can not change states until the opponent releases them.

Hypnosis and Psybeam are the only Psychic moves that have regular effects on ghost-type Pokémon.