In Felarya we tend to confuse the two terms quite a lot, often interchanging forest and jungle rather carelessly. Felarya certainly has quite it's share of both, but I think people should know the difference between the two, because they are actually, in fact, quite different from each other.
Some of you probably know this already, but I figured this is important enough of a distinction and a mistake to make this thread about it.
First off, Forests are cooler, and Jungles are typically hotter. This is not only because jungles are situated in tropical areas, but also because of how they keep heat. Anyone ever heard of a cool jungle? A jungle has a lot of plant life really close together, including trees, bushes, ferns, vines all sorts. The basic idea is that the sun shines down on top of the trees, and the heat is trapped under the canopies. Where as in a forest, even though trees can be quite close together, they are often in clumps. One clump over there, followed by a tiny open space, followed by another large clump. etc. Sunlight can shine down through the canopies, unlike in a jungle. Yes, for those of you who didn't know, sunlight cannot penetrate jungles effectively. There are occasional streams of light that keep the place bright enough to see, but for the most part is blocked out.
Jungles are also in smaller patches, found at the edges of forests. A jungle wont randomly appear in the middle of a forest just because the forest is extra dense. Jungles have specific plants that are more dense, and generally smaller than forest trees. They need a humid, warm area.
Forests can be any density, they grow everywhere including a hot or humid environment. The plant life is less dense on average, and also much taller. Forests contain all types of trees but not many varieties of trees are found in a single forest.
This brings to mind certain areas of the Felarya map. Would the Grove of Carnivorous plants be considered a forest or a Jungle? What makes the Jungle of Perils a jungle other than its name? Personally I'm not sure if it is humid or hot enough to be one, being right next to a massive river, but often the wetter a jungle is, the more humid it is, and water has very little effect on cooling it down (I think).
Felarya doesn't have an equator or an axis. There isn't any particular point that receives more sun than another spot. So what does this mean for jungles? Would Felarya actually be one big jungle? or one big forest. If there's a volcano nearby, and it's constantly erupting, then there might be a jungle around it due to the heat. Does all of felarya take heat from one sun? or do different areas take heat from different suns, or a place where the sun is bigger and hotter, or the planet is closer to the sun?
The part of the Jewel River where the Jungle is might open up a bit more in reality, and have a pretty long beach all around it's edge with tropical waters.
So there you have it. The main difference is density, humidity, size of plants, and global placement.