Azaleas are flowering shrubs making up part of the genus Rhododendron. Originally azaleas were classed as a different genus of plant, but now they are recognized as two of the eight subgenera of rhododendrons – Pentanthera (deciduous) and Tsutsuji (evergreen). Azaleas bloom in spring, in damp mountainous places, their flowers dying only a few weeks later. They do not need as much sun as other plants; they live near trees and sometimes under them.
One major difference between azaleas and the rest of the rhododendron genus is their size. Another is their flower growth; rhododendrons grow their flowers in stripers,[clarification needed] while most azaleas have terminal blooms (one flower per flower stem). However, they have so many stems that during the flowering season they are a solid mass of colour. Azaleas are recognized by these flowers blooming all at once, in a showy display for a month or two in spring. The exception to this rule is a small group of azaleas which grow their flowers in tight terminal clusters.
(Excerpt from WIKIPEDIA)