Uphill ski refers in this case to the back end of the ski, ie behind the skier.
Downhill ski refers to the front end of the ski. ie infront of the skier
The Snowplough turn (also known as the wedge turn or stem turn) is a development of the stem technique, where the uphill ski is stemmed or pushing out from being parallel with the downhill ski to form a V shape.
Both techniques described below were included in Mathias Zdarsky's book Lillienfeld Skilaufer Technik in 1896.
The Snowplough or Wedge involves forming a V shape with the skis. By applying pressure against the snow with the edges of the skis - taking care not to cross the tips of the skis - speed can be controlled or brought to a stop.
Having brought the skis into the V shape mentioned above, more pressure is applied to one of the skis, so causing the skis to turn (to the left if applied to the right ski, or to the right if applied to the left.)