To understand the difference between a rainforest and a jungle, we first look at the definitions of “forest” and “jungle”. This is not as easy as it would appear, since there are over 250 definitions for forests, alone! A jungle is generally defined as something that is densely overgrown or an impenetrable thicket of tropical or non tropical vegetation, which are usually incorporated as parts of forests. In other words, the definitions of forest and jungle are dependant upon the interest or the focus of the individual who is studying various aspects of these biomes.
A geologist, for example, might use some factors in classifying areas of flora, while a biologist might have another set of factors and a lawyer or corporation might use another set of factors.
A general definition of a forest and all of its living components will include, of course, the trees, but will also include the other grasses, plants, and flora, fungi, fauna, insects, and even the lifeforms in the water that make up the entire forest system.
This way, it is easy to see how a jungle, which is defined as the dense forest, or overgrown, uncultivated land, and impenetrable segments of vines, lianas, herbaceous plants, small trees, and shrubs would be incorporated into the overall defined rainforest as a section of the various levels of living material, but that jungles can and do exist independently of forests!
One factor in whether there can even be a combination of rainforest and jungle is that the trees in a rainforest can create a dense canopy layer which deprives other plant life of the sunlight that is required for survival and growth. Only so many full grown trees can survive to reach their great hight, making a rainforest a place where it is easy to walk due to the lack of dense undergrowth. When there is a threat to the canopy, and sunlight is allowed to penetrate to the ground, then a jungle can form, as the vines, shrubs, smaller trees and other plant life erupt to create a dense and established thicket.
As a result, all rainforests are jungles in one form or another, but not all jungles are rainforests, because any densely foilated area can be termed a jungle. The largest and most famous jungles of the world, however, are all contained within the defined tropical rainforest areas, but only thrive as established jungles where the forest canopy is not dense enough to cut off sunlight.
The Earth’s Tropical rainforests exist between the Tropic of Cancer and the Tropic of Capricorn which are near the equator. A rainforest is a forest that receives between 68 and 78 inches, or 1750 to 2000mm of annual rainfall. This massive amount of rain is generally confined to the monsoonal trough or the intertropical convergence zone, where the wind patterns of the Northern and Southern Hemispheres meet. If the winds go Westward, they are monsoonal. If they go Eastward, they are trade winds.
The Earth’s Temperate rainforests exist in the Temporate, or Tepid latitudes, which lie between the tropic latitudes and the poles of the Earth. These latitudes have a wide range, from humid and sub tropical, to extreme variants with winter and summer extremes in temperature. Thus, not all temperate rainforests contain jungles.
Wikipedia Temporate latitudes
An Interesting discussion of the evolution of a rainforest jungle, NY Times, 2009