Skiing pants come in two varieties: padded and shell. Shell trousers can be worn from fall till spring with the right base and mid layers, while padded ones are specialized for freezing conditions.
What makes the difference between trekking shell pants and skiing shell pants? First of all, skiing shell pants have wider cut legs to make room for skiing boots. Secondly, snow gaiters are common in skiing shell pants. Thirdly, skiing shell pants are never made with ultra light fabrics, as they tend to receive somewhat rough treatment.