Lofting defines surfaces that project between multiple curves. A lofted surface is used to create a smooth surface from cross-sectional curve data. The curves can be open or closed, but for closed curves the start points for each curve need to line up with the other curve start points for good results. The curves can be non-planar as well. See Twists in surfaces or solids with closed cross sections for more information.
Lofted surfaces are commonly used when there are many uniformly spaced cross-section curves available. A lofted surface has to fill in a lot of data between the cross-sections and so is an approximate surface. The data calculated to pass through the surface can be tweaked with the Uneven spacing switch for a better fit when the data points are not uniformly spaced.
Spline approximates the surface with input curves as control points/curves. They are smooth between points. Interpolate uses the input curves as explicit curves for the surface to pass through and the surface may wave, or bend between points. Setting the Uneven spacing switch might improve the fit if the input curves are not uniformly spaced. A lofted surface may be either exact or approximate depending on your settings.
The Select degree spinner allows you to vary the tightness by changing the degree of the polynomial used to calculate the resulting surface. A degree of one passes straight lines between curves (like a ruled surface). Higher degree curves allow a looser result between input curves. The highest degree possible is three or one less than the total number of curves, whichever is greater. If you are going to export the part file to another CAD package, some other software does not support degree values higher than three.
Chained or joined curves behave differently in a lofted surface than spline curves. If your surface does not look quite right in the Preview, set the Reparameterize curves check box and try the Preview again. Reparameterize curves analyzes the curves and may adjust the control points of the curves to yield a better surface result. Reparameterize affects both chained and spline curves, but the effect is stronger on chained curves.