Impacted wisdom teeth fall into one of several categories:
Mesioangular impaction is that the commonest kind, and means that the tooth is angulate forward, towards the front of the mouth.
Vertical impaction happens when the formed tooth doesn't erupt absolutely through the gum line.
Distoangular impaction means that the tooth is angulate backward, towards the rear of the mouth.
Horizontal impaction is that the least common kind, that happens once the tooth is angulate absolutely ninety degrees sideways, growing into the roots of the second molar.