An oak is a tree that grows in the Northern Hemisphere. There are deciduous (leaf-shedding) oaks and evergreen oaks. They can be found from cool to tropical regions. North America contains the largest number of oak species.

Oaks have very distinctive leaves made of a number of lobes. In some kinds of oak these are jagged as well. In spring, each oak tree produces both male flowers (as catkins) and small white female flowers. When the female flowers become pollinated, a fruit forms below the petals. The fruit is called a nut because it is hard. In this case it is acorn. The acorn grows in a kind of cup known as a cupule; each acorn contains one seed and takes 6–18 months to mature.

Oaks are vital to most forests where they grow because the acorns feed so many animals. The rough bark is also home to many insects, while holes in the trunk can be home to birds including owls. Many animals feed on acorns once they have fallen to the ground. These include squirrels, who rush away to bury the nuts as food for the winter. This is important, because otherwise the heavy acorns would all fall below the tree and never grow in its shade. Because squirrels forget where they have buried every single nut, some will grow up as small oak saplings the next year.