They are deciduous shrubs or small trees, ranging in size from 2–10 m tall, with stems up to 20–30 cm diameter. The leaves are opposite (occasionally in whorls of three) in arrangement, and their shape is simple and heart-shaped to broad lanceolate in most species, but pinnate in a few species (e.g. S. protolaciniata, S. pinnatifolia). The flowers are produced in spring, each flower being 5–10 mm in diameter with a four-lobed corolla, the corolla tube narrow, 5–20 mm long; they are asexual, with fertile stamens and stigma in each flower. The usual flower colour is a shade of purple (often a light purple or lilac), but white and pale pink are also found. The flowers grow in large panicles, and in several species have a strong fragrance. Flowering varies between mid spring to early summer, depending on the species. The fruit is a dry, brown capsule, splitting in two at maturity to release the two winged seeds.
The genus is most closely related to Ligustrum (privet), classified with it in Oleaceae tribus Oleeae subtribus Ligustrinae.
Lilacs are popular shrubs in parks and gardens throughout the temperate zone. In addition to the species listed above, several hybrids and numerous cultivars have been developed. The term French lilac is often used to refer to modern double-flowered cultivars, thanks to the work of prolific breeder Victor Lemoine.
Lilacs flower on old wood, and produce more flowers if unpruned. If pruned, the plant responds by producing fast-growing young vegetative growth with no flowers, in an attempt to restore the removed branches; a pruned lilac often produces few or no flowers for one to five or more years, before the new growth matures sufficiently to start flowering. Unpruned lilacs flower reliably every year. Despite this, a common fallacy holds that lilacs should be pruned regularly. If pruning is required, it should be done right after flowering is finished, before next year's flower buds are formed. Lilacs generally grow better in slightly alkaline soil.
Lilac bushes can be prone to powdery mildew disease, which is caused by poor air circulation.
The wood of lilac is close-grained, diffuse-porous, extremely hard and one of the densest in Europe. The sapwood is typically cream-coloured and the heartwood has various shades of brown and purple. Lilac wood has traditionally been used for engraving, musical instruments, knife handles etc. When drying, the wood has a tendency to be encurved as a twisted material, and to split into narrow sticks. The wood of Common Lilac is even harder than for example that of Syringa josikaea.
A pale purple colour is generally known as lilac after the flower.
Numerous locations around North America hold yearly Lilac Festivals, the longest-running of which is the one in Rochester, New York. Rochester's Lilac Festival held at Highland Park has the most varieties of lilacs at any single place and many of the lilacs were developed in Rochester. Spokane, Washington, is known as the lilac city, and holds a lilac festival every year, complete with a lilac parade.
"When Lilacs Last in the Dooryard Bloom'd" is a poem written by Walt Whitman as a elegy to Abraham Lincoln.