Wisdom teeth are so varied in how they present. Here is a patient I just saw and you can see the wisdom tooth on one side is fully impacted (impacted is when there is tooth or bone in the way stopping the tooth from erupting properly). The other side shows a partialy impacted wisdom tooth.
Unfortunately, the partial impacted tooth was visible in the mouth, so it had decayed. The tooth next to it had also decayed because food trapped between the teeth. We removed this partial impacted tooth today without incident.
See my other posts on wisdom teeth from February 2008 link.