There are a lot of items to bring with you when you sign up for an outdoor adventure, be it mountaineering or trekking.
In general, mountain climbing is an equipment intensive activity. Having good equipment in the mountains increases safety and your comfort level and therefore your chance of having a successful climb. At a minimum, a mountaineer needs a basic set of equipment and appropriate apparel in order to climb successfully and survive in the harsh outdoor conditions.
Trekkers are exposed to much less extreme weather conditions, so this activity requires a less extensive list of equipment and gear. Layering and flexibility are key to enjoying your trekking adventures, allowing you to easily accommodate your apparel to the changing weather patterns.