A wellbore is the hole created for the extraction of hydrocarbons. Wellbores are sometimes referred to as "drillholes," "boreholes" or "holes."
In a vertical well, the wellbore will be drilled in a relatively straight line directly below the surface. For a horizontal well, the wellbore will usually start in the vertical plane and at some point below the surface bend until the wellbore is roughly parallel to the surface.
For any point along the wellbore, the measured depth will be the distance along the wellbore while the true vertical depth will be the distance from the surface. For a vertical well, these two measures will be equal.