Most maples are trees growing to a height of 10–45 m (33–148 ft). Others are shrubs less than 10 meters tall with a number of small trunks originating at ground level. Most species are deciduous, and many are renowned for their autumn leaf colour, but a few in southern Asia and the Mediterranean region are evergreen. Most are shade-tolerant when young and are often riparian, understory, or pioneer species rather than climax overstory trees. There are a few exceptions such as sugar maple. Many of the root systems are typically dense and fibrous, inhibiting the growth of other vegetation underneath them. A few species, notably Acer cappadocicum, frequently produce root sprouts, which can develop into clonal colonies.
Maples are distinguished by opposite leaf arrangement. The leaves in most species are palmate veined and lobed, with 3 to 9 (rarely to 13) veins each leading to a lobe, one of which is central or apical. A small number of species differ in having palmate compound, pinnate compound, pinnate veined or unlobed leaves. Several species, including Acer griseum (paperbark maple), Acer mandshuricum (Manchurian maple), Acer maximowiczianum (Nikko maple) and Acer triflorum (three-flowered maple), have trifoliate leaves. One species, Acer negundo (box-elder), has pinnately compound leaves that may be simply trifoliate or may have five, seven, or rarely nine leaflets. A few, such as Acer laevigatum (Nepal maple) and Acer carpinifolium (hornbeam maple), have pinnately veined simple leaves.
Maple species, such as Acer rubrum, may be monoecious, dioecious or polygamodioecious. The flowers are regular, pentamerous, and borne in racemes, corymbs, or umbels. They have four or five sepals, four or five petals about 1 – 6 mm long (absent in some species), four to ten stamens about 6 – 10 mm long, and two pistils or a pistil with two styles. The ovary is superior and has two carpels, whose wings elongate the flowers, making it easy to tell which flowers are female. Maples flower in late winter or early spring, in most species with or just after the appearance of the leaves, but in some before the trees leaf out.
Maple flowers are green, yellow, orange or red. Though individually small, the effect of an entire tree in flower can be striking in several species. Some maples are an early spring source of pollen and nectar for bees.
The distinctive fruits are called samaras, "maple keys", "helicopters", "whirlybirds" or "polynoses". These seeds occur in distinctive pairs each containing one seed enclosed in a "nutlet" attached to a flattened wing of fibrous, papery tissue. They are shaped to spin as they fall and to carry the seeds a considerable distance on the wind. People often call them "helicopters" due to the way that they spin as they fall. During World War II, the US Army developed a special air drop supply carrier that could carry up to 65 pounds (29 kg) of supplies and was based on the Maple seed. Seed maturation is usually in a few weeks to six months after flowering, with seed dispersal shortly after maturity. However, one tree can release hundreds of thousands of seeds at a time. Depending on the species, the seeds can be small and green to orange and big with thicker seed pods. The green seeds are released in pairs, sometimes with the stems still connected. The yellow seeds are released individually and almost always without the stems. Most species require stratification in order to germinate, and some seeds can remain dormant in the soil for several years before germinating.
The genus Acer together with genus Dipteronia are either classified in a family of their own, the Aceraceae, or else classified as members of the family Sapindaceae. Recent classifications, including the Angiosperm Phylogeny Group system, favour inclusion in Sapindaceae. When put in family Sapindaceae, genus Acer is put in subfamily Hippocastanoideae.
The genus is subdivided by its morphology into a multitude of sections and subsections.
Fifty-four species of maples meet the International Union for Conservation of Nature criteria for being under threat of extinction in their native habitat.
Maple syrup is a syrup usually made from the xylem sap of sugar maple, red maple, or black maple trees, although it can also be made from other maple species. In cold climates, these trees store starch in their trunks and roots before winter; the starch is then converted to sugar that rises in the sap in late winter and early spring. Maple trees are tapped by drilling holes into their trunks and collecting the exuded sap, which is processed by heating to evaporate much of the water, leaving the concentrated syrup.
Maple syrup was first collected and used by the indigenous peoples of North America, and the practice was adopted by European settlers, who gradually refined production methods. Technological improvements in the 1970s further refined syrup processing. The Canadian province of Quebec is by far the largest producer, responsible for 70 percent of the world's output; Canadian exports of maple syrup in 2016 were C$ 487 million (about US$ 360 million), with Quebec accounting for some 90 percent of this total.
Maple syrup is graded according to the Canada, United States, or Vermont scales based on its density and translucency. Sucrose is the most prevalent sugar in maple syrup. In Canada, syrups must be made exclusively from maple sap to qualify as maple syrup and must also be at least 66 percent sugar. In the United States, a syrup must be made almost entirely from maple sap to be labelled as "maple", though states such as Vermont and New York have more restrictive definitions.
Maple syrup is often used as a condiment for pancakes, waffles, French toast, oatmeal or porridge. It is also used as an ingredient in baking and as a sweetener or flavouring agent. Culinary experts have praised its unique flavour, although the chemistry responsible is not fully understood.
A maple leaf is on the coat of arms of Canada, and is on the Canadian flag. The maple is a common symbol of strength and endurance and has been chosen as the national tree of Canada. Maple leaves are traditionally an important part of Canadian Forces military regalia, for example the military rank insignia for generals use maple leaf symbols. There are 10 species naturally growing in the country, with at least one in each province. Although the idea of the tree as a national symbol originally hailed from the province of Quebec where the sugar maple is significant, today's arboreal emblem of Canada rather refers to a generic maple. The design on the flag is an eleven-point stylization modeled after a sugar maple leaf (which normally bears 23 points).
It is also in the name of Canadian ice hockey club Toronto Maple Leafs.
The first attested use of the word was in 1260 as "Mapole", and it also appears a century later in Geoffrey Chaucer's Canterbury Tales, spelled as "mapul".
The maple is also a symbol of Hiroshima, ubiquitous in the local meibutsu.