Molecular cloud
A molecular cloud is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen (H2).
Molecular hydrogen is difficult to detect by infrared and radio observations, so the molecule most often used to determine the presence of H2 is CO (carbon monoxide). The ratio between CO luminosity and H2 mass is thought to be constant, although there are reasons to doubt this assumption in observations of some other galaxies.

# Occurrence
Within our own galaxy molecular gas accounts for less than one percent of the volume of the interstellar medium (ISM), yet it is also the densest part of the medium comprising roughly one-half of the total gas mass interior to the Sun's galactic orbit. The bulk of the molecular gas is contained in a molecular ring between 3.5 to 7.5 kiloparsecs from the centre of the galaxy (the Sun is about 7.6 kiloparsecs from the center). Large scale carbon monoxide maps of the galaxy show that the position of this gas correlates with the spiral arms of the galaxy. That molecular gas occurs predominantly in the spiral arms argues that molecular clouds must form and dissociate on a timescale shorter than 10 million years - the time it takes for material to pass through the arm region.
Vertically, the molecular gas inhabits the narrow midplane of the Galactic disc with a characteristic scale height of approximately 5075 parsec, much thinner than the warm atomic (Z=130-400pc) and hot ionized (Z=1000pc) gaseous components of the ISM. The exception to the ionized gas distribution are HII regions which are bubbles of hot ionized gas created in molecular clouds by the intense radiation given off by young massive stars and as such they have approximately the same vertical distribution as the molecular gas.
This smooth distribution of molecular gas is averaged out over large distances, however the small scale distribution of the gas is highly irregular with most of it concentrated in discrete clouds and cloud complexes.

# Types of molecular cloud

## Giant molecular clouds (GMCs)
Vast assemblages of molecular gas with masses of 104106 times the mass of the sun are called Giant molecular clouds (GMC). The clouds can reach tens of parsecs in diameter and have an average density of 102103 particles per cubic centimetre (the average density in the solar vicinity is one particle per cubic centimetre). Substructure within these clouds is a complex pattern of filaments, sheets, bubbles, and irregular clumps.
The densest parts of the filaments and clumps are called "molecular cores", whilst the densest molecular cores are, unsurprisingly, called "dense molecular cores" and have densities in excess of 104106 particles per cubic centimeter. Observationally molecular cores are traced with carbon monoxide and dense cores are traced with ammonia. The concentration of dust within molecular cores is normally sufficient to block light from background stars such that they appear in silhouette as dark nebulae.
GMCs are so large that "local" ones can cover a significant fraction of a constellation such that they are often referred to by the name of that constellation, e.g. the Orion Molecular Cloud (OMC) or the Taurus Molecular Cloud (TMC). These local GMCs are arrayed in a ring around the sun called the Gould Belt. The most massive collection of molecular clouds in the galaxy, the Sagittarius B2 complex, forms a ring around the galactic centre at a radius of 120 parsec. The Sagittarius region is chemically rich and is often used as an exemplar by astronomers searching for new molecules in interstellar space.

## Small molecular clouds
Isolated gravitationally bound small molecular clouds with masses less than a few hundred times the mass of the sun are called Bok globule. The densest parts of small molecular clouds are equivalent to the molecular cores found in GMCs and are often included in the same studies.

## High-latitude diffuse molecular clouds
In 1984 IRAS identified a new type of diffuse molecular cloud. These were diffuse filamentary clouds that are visible at high galactic latitudes (looking out of the plane of the galactic disc). These clouds would have a typical density of 30 particles per cubic centimeter.

# Processes

## Star formation
It is believed that the creation of newborn stars occurs exclusively within molecular clouds. 
This is a natural consequence of their low temperatures and high densities, since the gravitational force acting to collapse the cloud may exceed the internal pressures that are acting "outward" to prevent a collapse. Also there is observed evidence that the large, star-forming clouds are confined to a large degree by their own gravity (like stars, planets, and galaxies) rather than external pressure (like clouds in the sky).
The evidence comes from the fact that the "turbulent" velocities inferred from CO linewidth scale in the same manner as the orbital velocity (a virial relation).

## Physics
The physics of molecular clouds are poorly understood and much debated. 
Their internal motions are governed by turbulence in a cold, magnetized gas, for which the turbulent motions are highly supersonic but comparable to the speeds of magnetic disturbances. This state is thought to lose energy rapidly, requiring either an overall collapse or a steady reinjection of energy.  
At the same time, the clouds are known to be disrupted by some processmost likely the effects of massive starsbefore a significant fraction of their mass has become stars.
Molecular clouds, and especially "Giant" molecular clouds (GMCs), are often the home of astronomical masers.