Betulaceae, the birch family, includes six genera of deciduous nut-bearing trees and shrubs, including the birches, alders, hazels, hornbeams, hazel-hornbeam, and hop-hornbeams numbering a total of 167 species. They are mostly natives of the temperate Northern Hemisphere, with a few species reaching the Southern Hemisphere in the Andes in South America. Their typical flowers are catkins and often appear before leaves.
Betulaceae
Catkins of the hazel (Corylus avellana)
Corylus avellana foliage and nuts
A nut is a fruit consisting of a hard or tough nutshell protecting a kernel which is usually edible. In general usage and in a culinary sense, a wide variety of dry seeds are called nuts, but in a botanical context "nut" implies that the shell does not open to release the seed (indehiscent).
Chestnuts are both botanical and culinary nuts.
Nuts being sold in a market
Raw mixed nuts, sold as a snack food
Image: Almonds in shell, shell cracked open, shelled, blanched