Trekking is an activity of walking the natural landscapes (especially the side of the mountain) with a specific purpose of exploring and enjoying the scenery. Trekking is generally a multi-day walk, where you stay overnight at trails.
It tests the trekker's physical ability, stamina, endurance, and also the psychological capacity. Trekking is different from hiking as trekking is a rigorous and challenging activity, whereas hiking is mainly a leisure activity. Hiking generally ranges from a single day walk.
Climbing, on the other hand, is a much more challenging that requires various technical skills like abseiling, rope fixing, among others. A climber is much riskier requires specialized equipment, as well as rigorous training.