Hiking is a great activity for everyone young and old.
For the young it allows great quality time with parents and friends. For everyone, it is great and healthy exercise, when proper trails are chosen in conjunction with your athletic ability, general health, and general enjoyment proclivities. Hiking can be classified many different ways, I've chosen some simple differentiations since these pages are intended as an introduction to the hobby with pointers to some very interesting places.
Day Hikes
I've covered some nice trails in Yosemite National Park (US), Maui, Hawaii and the Engadin area of Switzerland. This type of activity can be accomplished by almost anyone. The places mentioned are among the most beautiful that I've been to. They are chosen as a representative sample of hiking in it's most elementary and easy form. Try some of these and soon you will be moving up to overnight trips etc.
Hiking and trekking on a guided basis
I've covered several adventures of this nature. Read about Thailand, Alberta, Canada and - when I've finished it - New Zealand

This type of activity while perhaps beyond the physical abilities of some, is safe and enjoyable if you can take the pace. Trip usually involve overnight stays and the ability to carry a small day pack with munchies, water and misc items for uses between camp sites. In some cases, as in Nepal they are extremely stenuous and may require a heavier pack
Backpacking and more strenuous forms of hiking
The sheer joy of walking into country of astounding beauty and loveliness, leaving behind the noises of advanced civilization, travelling at your own chosen pace, getting tired (but relaxed tired), and ending a day at your own campsite, cooking and a good book or good conversation around a fire cannot be conveyed w/o experiencing it.