According to NASA, Aquarius "is a focused effort to measure Sea Surface Salinity and will provide the global view of salinity variability needed for climate studies." A Delta II rocket lifted off from Vandenberg Air Force base carrying the Aquarius/SAC-D spacecraft. Within an hour it had "separated from the rocket's second stage and began communicating with ground controllers and unfurling its solar arrays."
The mission of Aquarius is to investigate the salinity of the oceans. Within a few months, "Aquarius will collect as many sea surface salinity measurements as the entire 125-year historical record from ships and buoys." These data will be invaluable in the investigation of the water cycle, ocean circulation, and climate change.
Regarding the climate aspects, the blog RealClimate, run by a group of world renowned climatologists, has a great summary of why the data Aquarius collects is so important.
You can follow the progress of Aquarius on the NASA web site here.