What is Growing on These Tree Leaves? (Image)
This is actually not a gall as other answers have suggested. 
This is likely a fungus called Cedar-apple rust (Gymnosporangium juniperi-virginianae).

The fungus only thrives in the presence of both Juniperus virginiana (Eastern red cedar) and apple (Malus spp.) trees.

The leaf in the picture belongs to some species of the apple genus and the growths are aecia of G. juniperi-virginianae. You can read about this interesting fungi through Rutger's Plant &amp; Pest Advisory here.

You can see a picture of the fungi here, and I've included Sabrina Tirpak's (Rutgers PDL) photo of its aecia below:

http://plant-pest-advisory.rutgers.edu/3859/

Below is a visual of the telia of the rust fungus on J. virginiana:



 (Photo source: Ada Hayden Herbarium)