The Himalayas are known to be young fold mountains. Young, because these have been formed relatively recently in the earth's history, compared to older mountain ranges like the Aravallis in India, and the Appalachian in the USA. They are known as fold mountains because the mountains extend for 2500 km in length in a series of parallel ridges or folds.
The Himalayan mountains began forming in the late Cretaceous-Tertiary Period (70my) and continue to increase in elevation even today. That makes them among the youngest of mountain chains on the earth. They are fold mountains because of the compressive forces involved in the collision of the two continental plates involved, the Asian and the Indian/Australian.

Appalachian mountains are called old folded mountain but Himalayas are called new folded mountains.