Different counties have different methods for ASSESSING a home's value. (as others have said, it's NOT an appraisal).
As a homeowner, and you disagree with your county's assessed value (feel it's valued too high and you're paying too much in taxes), then you can try to contest it. (Sometimes it works, sometimes it doesn't). But you need to know how they arrived at that figure to begin with and be able to provide back up information as to why their value is incorrect. Look on your county's website.
But as a buyer or seller, I wouldn't pay ANY attention to the assessed value. Have your REALTOR do a CMA.