The greenhouse effect is a naturally occurring phenomenon that is responsible for heating of earth's surface and atmosphere. Clouds and gases reflect about one-fourth of the incoming solar radiation and absorb some of it but almost half of incoming solar radiation falls on earth's surface heating it, while a small proportion is reflected back. Earth's surface re-emits heat in the form of infrared radiation but part of this does not escape into space as atmospheric gases (eg $\mathrm{CO}_2, \mathrm{CH}_4$ etc) absorb a major fraction of it. The molecules of these gases radiate heat energy and a major part of which again comes to earth's surface thus heating it up once again. $\mathrm{CO}_2(60 \%)$ and $\mathrm{CH}_4(20 \%)$ are commonly known as greenhouse gases because they are responsible for the greenhouse effect, that ultimately leads to global warming.