Water vapor is a core greenhouse gas with a major role in climate change

Water vapor and CO2 - American Chemical Society
http://www.acs.org/content/acs/en/climatescience/climatesciencenarratives/its-water-vapor-not-the-co2.html

Water vapor is the most abundant greenhouse gas, more abundant that CO2. However, a big outstanding question is whether water vapor has an overall effect of warming or cooling the atmosphere.

This is simply because
-on on hand water vapor as a greenhouse gas warms the atmosphere by trapping heat
-on the other water vapor as clouds reflects some of the sun's incoming radiation which actually cools the atmosphere

The article mentions that scientists are working on determining if these two balance out to either a warming or cooling effect overall.

Positive feedback
There is also a positive feedback effect: if the atmosphere is warming due to greenhouse gases including water vapor then more evaporation takes place, which then by definition puts adds water vapor i.e. greenhouse gases. To put it differently, water vapor as a greenhouse gas contributes to the production of more of itself.

Water vapor qua greenhouse gas > atmospheric warming > increased evaporation > more water vapor > repeat