A property tax is a levy on property that the owner is required to pay. Property taxes are generally levied by county, but often include taxes paid to other local entities like school districts, utility authorities or city governments. The rules, rates and regulations surrounding property taxes can vary significantly depending on where you live. 
Property tax appeal
If the appraised property value is very different from what you think it should be you can appeal the appraised value with your county. Normally there will be a hearing for so please prepare all the supporting materials before that.
Note that the appraised property value is different from the appraised value by your mortgage providers.