From This Big City:
"Since water has an ability to store heat or cold, these aquifers work a bit like a thermos. The idea, more or less, is to pump up cold water in the summer to cool buildings above ground. This makes the water temperature rise a small amount. This water is then pumped back down into the ground and stored until next winter, when it can be used for heating buildings. In total, this process generates about three or four times as much energy than what is required for pumping the water up and down."
According to a post at ThisBigCity.net, scientists in Sweden "have yet to find any disadvantages with this technology."