What are greenhouse gases? How are they responsible for global warming?

Carbon dioxide, water vapour, oxides of nitrogen, methane, ozone, chloro-fluoro carbons, etc. are called greenhouse gases.

The greenhouse gases in the atmosphere allow the infrared rays from the Sun to penetrate the Earth but do not allow the heat radiated by the heated Earth to penetrate back into space. This trapped radiation warms the earth's surface and lower parts of the atmosphere leading to increase in temperature which is known as global warming.