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