Greenhouse gases cause the Greenhouse effect, which is a naturally occurring phenomenon, responsible for the heating of the earth's surface as well as atmosphere. Major greenhouse gases are carbon dioxide and methane.
Greenhouse gases allow short wavelength radiations (like UV radiations) to come in but does not allow long-wave radiations to go out, so they absorb long-wave (infrared) radiations from the earth and emit it again towards the earth. This cycle continues till there are no long-wave radiations left to emit from the earth's surface. Resultantly, global warming is caused.