Not ALL base's are sintered. Many are extruded. Both are polyethelene. All start the process as extruded. Extruded base material is less expensive, softer , less durable has a lower molecular density (weight) and absorbs less wax. Sintered base material has a higher molelecular density (UHMW - ultra high molecular weight). It is more durable, absorbs wax better, is faster and more expensive.
As posted above, AT, Tele, Nordic and Alpine skis all have polyethelene bases. All require glide wax. Wax often. When using skins, don't scrape as much as you would for a lift served pair of skis. As mentioned previously, the wax helps keep the skin adhesive from coating the ski base.