Net metering is a billing mechanism that credits solar system owners for the electricity they generate and add to the grid. Most homes with a solar system will generate more electricity during the day time (when the sun is up) than they consume. Net metering programs allow for the solar system owner to earn credit for this extra production towards future use.
In other words, Net Metering programs are designed to allow the solar system owners to get credit for all the energy they produce even if they are not consuming all that energy at the time of generation. Net Metering rules and credits vary depending on your utility company. For example, in most locations in California, solar system owners earn full retail value for the solar energy they produce. For further information on Net Metering, you can have a look at this page.
Recently, in Hawaii, Net Metering rules changed crediting system owners at a lower value than the full retail rates.
The major utilities in Arizona and California recently proposed changes to substantially decrease the amount of Net Metering credits. However, the California Public Utilities Commission recently announced a proposed decision which kept the existing Net Metering rules in tact for the most part.