Hollies are shrubs and trees from 2–25 m tall, with a wide distribution in Asia, Europe, north Africa, and North and South America. The leaves are simple, and can be either deciduous or evergreen depending on the species, and may be entire, finely toothed, or with widely-spaced, spine-tipped serrations. They are mostly dioecious, with male and female flowers on different plants, with some exceptions. Pollination is mainly by bees and other insects. The fruit is a small berry, usually red when mature, with one to ten seeds.

Holly berries are mildly toxic and will cause vomiting and/or diarrhea when ingested by people. However they are extremely important food for numerous species of birds, and also are eaten by other wild animals. In the fall and early winter the berries are hard and apparently unpalatable. After being frozen or frosted several times, the berries soften, and become milder in taste. During winter storms, birds often take refuge in hollies, which provide shelter, protection from predators (by the spiny leaves), and food. The flowers are sometimes eaten by the larva of the Double-striped Pug moth (Gymnoscelis rufifasciata). Other Lepidoptera whose larvae feed on holly include Bucculatrix ilecella (which feeds exclusively on hollies) and The Engrailed (Ectropis crepuscularia). The Japanese Beetle (Popillia japonica) is another well-known animal feeding on holly leaves.

Having evolved numerous species that are endemic to islands and small mountain ranges, and being highly useful plants, many hollies are now becoming rare. Tropical species are especially often threatened by habitat destruction and overexploitation, and at least two have become extinct, with numerous others barely surviving.