Merocrine glands secrete hormones into the bloodstream without causing any harm to their secreting cells. Merocrine glands release hormones with the protoplasm (cell contents minus the nucleus, some cytoplasm, and some organelles). For example, the palm.
Apocrine glands are glands that release enough secretion to form buds to create new cells. For example, the mammary gland.
Holocrine glands typically operate by causing the membrane surrounding the cell to rupture and release the cytoplasm into the cavity within the organ. The sebaceous gland is an example of the holocrine gland.