The early botanist Linnaeus distinguished azaleas from rhododendrons by the five stamens that most azalea flowers have, while rhododendron flowers have ten. Carl Maximowicz corrected this mistake in 1870, but, as far as most gardeners are concerned, Linnaeus was right. Actually, all azaleas are rhododendrons. In recent years, however, rhododendron has been used to refer to the plants with large, leathery, evergreen leaves. Azalea refers to those with smaller, thinner leaves.
Some azalea varieties are deciduous, which means they lose their leaves in the winter.
Others are green year-round. Session II, Azalea Culture Forum, pp. Dirr, Michael A., and Charles W. Heuser, Jr. 1987. The Reference Manual of Woody Plant Propagation from Seed to Tissue Culture. Galle, Fred C. 1987.