Extrude creates a surface from a curve by extending that line sideways a specified distance. Sideways can be a linear distance in any direction. This is important as the extrude doesn’t have to be only in the direction of an X,Y or Z axis. Extruding along an axis is an easy way to build such a surface which you could then move or transform into the final position if you prefer. Extrudes are a shortcut to create a ruled surface between a curve and a transformation of the curve.
Extruded surfaces are exact.
An example of an extruded surface.