Like other engineered wood products, Medium Density Fiberboard, or MDF for short, has a distinctively flat, dense surface that holds paint well. It doesn't move like wood, so its joints stay tight and paint doesn't crack, but the glory of MDF is its uniformity; it can be machined into every conceivable shape to create architectural details such as balusters or moldings. Unlike real wood, MDF has no knots, grain or warping that can make intricate woodworking difficult. While solid wood is better suited to structural applications such as floor joists, MDF tends to be more cost effective than solid wood so it's well suited to interior moldings, millwork and cabinetry.
For additional information on MDF see the Primed MDF page of this site