A feed-in tariff (FIT) places a legal obligation on utilities to buy electricity from renewable energy projects. The tariff rate is guaranteed for a certain period of time, hopefully 20 years if it's a good setup. The tariff rate is scientifically determined for each technology to ensure profitable operation of the installation.
You're going to ask how that differs from net-metering now, aren't you? According to Solar Nation, the difference feed-in tariffs and net metering laws is that the former permit ordinary citizens to profit from their clean energy investment, while the latter simply reduces the net kilowatt usage and partially offsets the electricity bill of the homeowner.
via RenewableEnergyWorld.com and Solar Nation