An assessment is the value placed upon your real estate or personal property by the assessor. This value determines what portion of the local property tax levy will be borne by your property. They are based on the amount that a typical purchaser would pay for the property under ordinary circumstances.
The assessment represents the value of the property as of January 1 of a given year. After the assessment is completed, property owners are sent a notice of assessment. After receiving this notice, owners have a limited period of time to discuss the assessment with the assessor and appeal through the process of Open Book and Board of Review. This is important because the property tax is calculated based on this assessment. (NOTE: the notice will indicate value AND classification. In rural areas the classification can be important because of the different value placed on agricultural land, undeveloped land and forest land.)
For more information, the Wisconsin Department of Revenue has a Guide for Property Owners that answers questions about property assessment and taxation in Wisconsin.