Stars go through different processes during their pre-main sequence evolution. Stellar parameters like radius, temperature, density etc. suffer drastic changes
in different stages. Molecular clouds of few thousand astronomical unit(AU) contract to form stars of the size of fraction of AU whereas the temperature rises from 10-50 K to the order of 10^6 K and density increases ~10^20 times. I will mainly discuss different mechanisms that drive the process of contraction and finally forms a main sequence star.