Hiking is described as going for (long) walks in the countryside on trails/footpaths. Hiking can for example take place in forest, along beaches, on hills or in the mountains.
 
A multi-day hiking is often called a trekking and you will sleep at different places. This can be in a mountain hut, at campsites or in a hotel, hostel, Bed&Breakfast depending on where your trekking takes place. 
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Hiking in the mountains has also a positive effect on our physical and psychological well-being. Hiking up and down is sometimes physically and psychologically hard. But ones you have reached your goal you get hopefully rewarded with a beautiful view and maybe the feeling you have achieved something you thought you were incapable off. It gives you new motivation and helps you improving your self-confidence.