Hiking = a long walk, usually for pleasure, usually on one day.
Trekking = a long walk, usually over difficult terrain/rough country, sometimes taking several days. Comes from the Afrikaans for travelling in an ox wagon.
Climbing = To go up a hill, mountain, cliff-face.
Mountaineering = the activity of climbing mountains.
Orienteering = a competitive sport in which runners have to find their way across rough country with the aid of a map and compass.