When property taxes are being determined, the fair market value is actually determined by the tax assessor of the county. This process is not a simple one. However, it is derived using a combination of the amount of property owned, the location of the property and assigned value of properties in the area.
Appraisers, on the other hand, use information obtained from research of the market and the activity of buyers of other properties. They use this information to form an opinion of value or appraised value.