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 does not 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.
To create an extruded surface:
Example
The handle on this hairdryer is an extruded surface:
The curve for the handle was drawn with the arcs and lines tools. The geometry was chained into a single curve which was then transformed down in Z to the depth where the handle exists. This transform operation puts a bounding curve in location for the top surface of the handle as well.
Then the handle curve was extruded along a vector -0.75 inches in Z.