Also known as Quickthorn, May-tree, Whitethorn, May-flower.

Indigenous to the British Isles, Hawthorn is now more widely found throughout the Northern Hemisphere.
The limbs of the Hawthorn form a dense network of twigs and branches, many of which are thorned. Because of this they were used for centuries to mark land boundaries, as they form an almost impenetrable barrier.
Hawthorn produces clusters of 5-petaled white flowers, known as 'May' after the month in which it blooms. They also produce winter berries called Haws which are deep-red in Autumn, and are enjoyed by many birds such as redwings and fieldfares. 

Hawthorn is a pagan symbol of fertility and is closely linked with May Day, the celebration of the end of Spring and beginning of Summer. Hawthorn lent its leaves, thorns and flowers to many a May Day garland, as well as appearing in the wreath of the Green Man. 

The fair maid who, the first of May,
Goes to the fields at break of day
And washes in dew from the hawthorn tree,
Will ever after handsome be.

In Irish legend, the Hawthorn tree, lonely and desolate in the landscape as it was, was believed to be inhabited by Faeries. It could not be damaged in any way without fear of incurring the wrath of these faery guardians.

The Hawthorn is also steeped in another uniquely Anglo legend. It was associated by early Christians with the Saint, Joseph of Arimathea. Joseph was said to have laid his hawthorn staff to the ground of Glastonbury, England, and it miraculously produced a 'Holy Thorn' that blooms come Christmas. The thorn 'Biflora' (named so as it flowers twice a year) or 'Glastonbury Thorn' is still found in the area to this day. A flowering sprig of a descendant of this tree is sent to the British Monarch every Christmas.

Further reading and sources
Woodland Trust 
Trees for Life - Mythology and folklore of the Hawthorn