Trekking is not necessarily mountaineering, more walking for a number of days, usually on uncharted paths, in challenging environments which are likely to be hilly or mountainous. It's common to bring survival kit and camping gear and stay out for days. Common trekking paths can be the Himalayan foothills in Nepal, India and Bhutan. Or the Andes in South America. Trekking is a great way to explore the nature without leaving an environmental footprint, which means minimum impacting of the nature.