Holly forms a small evergreen tree or shrub up to 15m high, typically with a conical or cylindrical crown and the following features:

• bark is smooth and often silvery grey
• twigs are green
• alternate leaves are 3–10cm long, oval, elliptical or oblong, thick and leathery in texture, dark, glossy green on the upper surface, paler green on the lower surface
• leaf margins are usually wavy, with thickened and sharply spiny margins, but old trees often produce leaves with few if any spines

The trees are either male or female. In both sexes, the flowers are around 6mm in diameter and white, with 4 petals. They are borne in small clusters in the angles of leaf and twig on old wood - twigs more than one year old. Only the male flowers are fragrant.The berries, found only on female trees, are 7–12mm diameter, globose and scarlet when ripe. They contain 3 or more small stones.Ilex aquifolium and the closely related Ilex perado - native to the Azores, Madeira and Canary Islands - and Ilex platyphylla, also native to the Canary Islands, form a poorly-understood group.Collectively these species and their numerous varieties, cultivars and hybrids are often referred to as English hollies. Many of the varieties are very old and of uncertain origin. One of the earliest cultivars known is ‘hedgehog holly’, supposedly introduced into England from France in the late 17th century. One of the best-known hybrids is Highclere holly, Ilex x altaclarensis (Ilex aquifolium x Ilex perado), a tree which originated in the gardens of Highclere Castle, Hampshire, home of the Earls of Carnavon in about 1835.English hollies are extremely variable in shape, vigour and bark colour. They are also variable in the following ways:

• variegated varieties may have leaves either edged or spotted with white, silver or gold
• some varieties have very spiny leaves, others no spines at all
• others produce yellow berries rather than the more common red berries