A giant molecular cloud is a dense - on a cosmic scale - cloud of gas and dust, which is cold enough to allow more or less complex molecules to appear. The temperature of such a cloud is typically around 15 K, that is -258°C.
This cloud has a mass between 100.000 and a few million solar masses of material.
It's mainly constituted of hydrogen, but it also harbours complex organic molecules based upon carbon. These molecules are the basis of life, as we know it...
Source : NASA / HST
Following an external event, the shock wave of a nearby exploding star for instance, or the density wave of a spiral arm of the galaxy, some parts of this cloud will condense in regions which are called 'Barnard's objects' and 'Bok's globules'. These areas, where density is significantly growing along with the temperature, are called protostars.
Stars are very social objects at the beginning of their lives : they are born in groups within clusters, because a cloud can generate a great number of stars in a short - always in cosmic scale - period of time. Later, they'll move away to live in a more isolated manner.
By condensing, the gas of protostars will heat. When it is hot
enough, it will produce a radiation emission in the microwave
range, and then in the infrared.
At a temperature between 2000 and 3000 K, it will be able to radiate some red
light, but this light will be absorbed by the surrounding cloud of dust.
So, we can't directly see the forming star, but we can detect such areas by the infrared radiation of the surrounding cloud.
The collapse of the cloud induces at the same time a transition from a more or less spherical form
towards a disc one :
this flattening is the result of the conservation of angular momentum.
Such a cloud is very large, it can be spread over an area as large as twenty solar systems.