Someone will give you a more technical answer, but to me, "trekking" = hiking + (fun, adventure or excitement). Hiking also means you walking. Trek doesn't necessarily imply walking. Hiking is more commonly used in everyday speech. I like to go hiking in the mountains. My brother took a short hike from the campsite this morning. Kevin trekked across the United States by bicycle. "And you people, you're all astronauts on ... some lind of star trek." - Zefram Cochrane, Star Trek First Contact

ok, I understand. For me is strange, because in Spain we use the word "trekking" to talk about a short walk. Is a word that we have imported to Spanish. Thank you!

Yeah, trekking kind of has this flavor of walking in foreign lands and the sense of adventure that comes with that. In the US, you'll here people say....I'm going trekking in Nepal or I have trekked through China...but you won't hear people use trek in this context....I am going trekking in my local park this weekend. (Use hiking instead)....and you won't even really hear people use it for places in the US that are far from them...For example, no one here would say...I trekked the Applachian Trail year. We would say I hiked the Applachian Trail last year.

Trekking implies a longer adventure... e.g. 'Trekking in the Himalayas' A hike is like a long walk you'd take at the weekend. (in my opinion)