Bone cyst. Cyst is usually consider benign.
Usually not. Most bone cysts are benign. Malignant bone tumors usually cause bone destruction and periosteal reaction on bone x-ray.
Nomenclature. All cancers are tumors, but benign tumors are the opposite of malignant tumors / cancers. The familiar (mismamed) "bone cysts" are benign; there's some disagreement among pathologists whether they are even tumors.
Depends. It depends on the type of bone cyst.