There are about 400 species of Holly (Ilex spp.) around the world, with about 80 of them considered to be threatened in the wild. So called 'English Holly" (Ilex aquifolium) is popular in temperate areas for its attractive red fruits against the dark glossy foliage.
These fruits are also attractive to birds who eat them and deposit seed elsewhere, often under other trees. As 'English Holly' is very shade tolerant, it is becoming invasive in forested areas where it is not native, including old-growth forests in the US.
In Ireland, where I. aquifolium is native, it has a strong association with winter and Christmas. In pre-christian times, Holly was seen as a source of life and growth in winter, when the red fruits and dark-green foliage were at their peak.
Holly berries are somewhat toxic to humans.