Ok, it's caused by the greenhouse effect which is caused by greenhouse gases such as CFC's, CO2, Methane and even water vapour.
As the temps rise the ice begins to melt at the poles, if the ice at the north pole melts it won't effect sea level because the ice is resting on water but if the ice in Greenland etc or the south pole melts the sea levels will rise.
The ice at the north pole is made up of fresh water, if that fresh water get into the salt water of the Atlantic is could cause the gulf stream to shut off. The gulf stream is a natural phenomenon that moves hot water and air from the Caribbean to the north Atlantic and northern Europe, it's possible if it was to shut off that north America and northern Europe could suffer from an ice age rather than warming.
In places such as Russia there are bogs in the ground which are currently storing huge amounts of methane gas under permafrost, if global warming was to melt this permafrost then this methane could be released (methane is one of the most potent greenhouse gases) and it could cause a speed up of the greenhouse effect.
If sea levels were to rise even by just a metre it would be devastating for countries such as the Netherlands. Click here for a map to study the effects of sea level rise: http://flood.firetree.net/
There have been ideas to try and tackle the increasing greenhouse gases in the atmosphere. Some scientists believe special kinds of algae could be engineered that would feed on CO2 in the atmosphere. That is just one of many ideas.
EDIT: Who the hell is giving me negative feedback? this is all scientific fact. Nowhere do I say global warming is or is not man made. Everything i've just wrote is completely accurate. Whoever gave me neg feedback must be completely thick.