Star formation begins when the denser parts of the cloud core collapse under their own weight/gravity. These cores typically have masses around 10⁴ solar masses in the form of gas and dust. The cores are denser than the outer cloud, so they collapse first. As the cores collapse they fragment into clumps around 0.1 parsecs in size and 10 to 50 solar masses in mass. These clumps then form into protostars and the whole process takes about 10 millions years. 
There are two common ways this can happen: it can either collide with another dense molecular cloud or it can be near enough to encounter the pressure caused by a giant supernova. Several stars can be born at once with the collision of two galaxies. 
Many of the most interesting infrared objects are associated with star formation. Stars form from collapsing clouds of gas and dust. As the cloud collapses, its density and temperature increase. The temperature and density are highest at the center of the cloud, where a new star will eventually form. The object that is formed at the center of the collapsing cloud and which will become a star is called a protostar. Since a protostar is embedded in a cloud of gas and dust, it is difficult to detect in visible light. Any visible light that it does emit is absorbed by the material surrounding it. Only during the later stages, when a protostar is hot enough for its radiation to blow away most of the material surrounding it, can it be seen in visible light. Until then, a protostar can be detected only in the infrared. The light from the protostar is absorbed by the dust surrounding it, causing the dust to warm up and radiate in the infrared. Infrared studies of star forming regions will give us important information about how stars are born and thus on how our own Sun and Solar System were formed. 
Websites:
http://www.ipac.caltech.edu/outreach/Edu/sform.html
http://abyss.uoregon.edu/~js/ast122/lectures/lec13.html
http://science.nasa.gov/astrophysics/focus-areas/how-do-stars-form-and-evolve/