What you should bring depends largely upon where, when and how you are trekking. The main stress while trekking is on keeping warm and dry while still being lightweight. At altitudes above 3,000 meter, you will need warm clothing at all times, while at lower altitudes, you will need to keep warm only in the evenings. And for lower altitudes, fleece jackets are adequate, while at higher altitudes down jackets are advisable.
Waterproof, windproof jackets and pants, well broken in footwear, head cover are also recommended. Good hiking shoes, a pair of sandals, sleeping bags, down jacket, sunglasses, warm woolen hiking socks, lightweight warm gloves, drinking water bottle, water purification tablets (which is highly recommended) a Swiss army knife, torchlight, sun block cream, toiletries, one quick drying towel, first aid kit and a very good route map are other things that come very handy. You also need a camera to take home memories of the trip.