Leaf cutter ants and fungus are an example of this relationship. The ants protect the fungi from pests and mold and also feed it with small pieces of leaves. The ants keep their larvae in the fungi which protects it and feeds it.
The strangler fig starts out life growing on a branch of a tree. It grows both downwards towards the ground and upwards to the sky, while also winding around the tree. The strangler fig kills the tree by stealing sunlight and root space after enveloping it.
Bromeliads, to get enough light, grow on high branches of trees. This does not do any damage to the tree itself, but it allows the brmeliad to survive.
In the dense tropical rainforests, there is fierce competition for sunlight. The taller, older trees that have established a spot in the forest shadow the ground. Due to that, new trees and other plants on the forest floor can't grow.