They are usually called galls. They don't always look like this; like Gremlins, they change when wet.
The cedar apple rust fungus cycle takes two years. Air-borne spores are released from infected apple leaves in late summer. Spores settle from the air onto junipers. Eastern red cedar, Juniperus virginiana, is a common host.
Wind carries the spores away. If they fall on apple leaves, they can germinate and infect the tree. Pale yellow spots appear on infected apple leaves. These "rust spots" mature by summer and eventually open to release spores that are carried by the wind to junipers. The cycle repeats.
The spores produced on apple leaves don't infect apple trees. The spores produced in juniper galls don't infect junipers. The fungus needs both hosts to live.