Hazel has both male and female flowers. The familiar yellow catkins are made up of about 250 male flowers. They produce the pollen; if you tap a ripe hazel catkin it will release a cloud of pollen. The female flower is a minutely small red tassel, somewhere on the same twig as the catkins.
Plants that produce separate male and female flowers are called monoecious and they usually have some way of preventing self fertilisation. Self fertilisation is a last resort kind of thing; in Britain for instance, bee orchids self fertilise because the species of bee that is supposed to carry pollen between the flowers doesn’t actually live here. If a plant can cross fertilise, it will find a way.
Hazel avoids self fertilisation with careful timing; the male catkins ripen first and when their pollen has been blown away on the wind, the tree’s female flowers open to wait for the pollen of a tree that is working to a different timetable. Each fertilised female flower will result in up to four hazelnuts.
male catkins ripen first 
a minutely small red tassel
Photographs by Suzanne Humphries
By this time of year, the female flowers on many mature hazel trees will be open. You may need to look carefully to find them them; they look like tiny sea anemones. If you look further up the branch to the catkins which have opened and dispersed their pollen, they are beginning to fade.
More about hazel:
Hazel Catkins
Southwick Country Park Nature Reserve 