Calendered materials have gone through an additional treatment process to flatten the fabric and seal the gaps between the individual fibers of the weave. The sealed weave adds higher wind resistance, the ability to hold in clusters of down and can improve hand-feel.
This process traditionally involves hot rollers and pressure, but can also include special coatings to insure that insulation cannot escape and that the fabric has the desired traits. Because of the heat involved, calendering does tend to decrease the tear strength of the fabric somewhat. So for a given fabric, there's a tradeoff between things like downproofness/wind resistance and tear strength when calendering.
Due to this process, you might notice one side of the material (usually used as the "back") is shinier and slicker.