Winter interest plants can be divided into four categories.
Evergreen plants provide year round structure to the garden, and conifers come into their own with tasteful hues of blue and green and twisting or classically upright habits.
Birch typifies the classic winter interest plant, but there are many other species with fantastic bark and stems. Willow and Dogwood both contain species with colourful stems, and are suitable for coppicing, which makes them ideal for small spaces. More exotic imports such as Acer palmatum, Eucalyptus and Tibetan Cherry are great, along with architectural specimens such as Corylus 'Contorta' and Salix 'Kilmarnock'.
Winter flowering plants are few and far between, but they make impressive specimens, and they do provide value to wildlife. Camellia japonica colours up right at the tail end of winter, but for mid-winter flowering plants Witch Hazel, Mahonia and Prunus 'Autumnalis' make excellent choices.
For long lasting berries, both Crabapple and Rowan trees' fruits last well into winter, but only Holly's fruits that will persist through the entire season.