A post box (or mailbox) built in the form of a short thick post. One drops one's letter into a pillar box so that a letter carrier will collect it. They are best known for their use by the UK's Royal Mail, although pillar boxes of various sorts can be found in any number of other countries around the word, from Spain to China.
The first English pillar boxes appeared in 1852 on the island of Guernsey; there were plenty of people wanting to send letters, but no post office. Anthony Trollope (who worked as a surveyor's clerk for the post office before he started writing) suggested constructing letter boxes, of the sort that were already common in France. A local blacksmith, John Vaudin, cast seven pillar boxes, one of which still survives on Union Street in St. Peter Port. These boxes were hexagonal, 4 feet 8 inches tall, and painted red.
They were so popular that they were quickly taken up on the mainland (can an island country have a mainland?) In 1859 a standard was put in place for new pillar boxes; they were to be cylinders, with a horizontal letter slit. They were also supposed to be a bronze-green color. Pillar boxes have also traditionally had either the royal cipher or the Latin initials of the current ruler on them, usually painted on in gold. (The first ones on Guernsey had the cipher of William IV.)
In 1874 the standard color was changed from green to red, the color for which pillar boxes are famous.
A quick note: In America we just call them all mailboxes, but in England there are all kinds of post boxes, including the wall box, the lamp box, and of course, the pillar box. The term letter box can apparently include all of these, in addition to mail slots.
References:
http://en.wikipedia.org/wiki/Pillar_box
Other Wikipedia.org pages, including the ones for the royal mail, letter box, lamp box, wall box, etc.
http://www.glias.org.uk/news/197news.html
http://www.shortal.com/kitmaster-bloke/PlacesToVisit.html
http://www.ausgang.com/collect/post.html
When you watch something that is not wide screen on a wide screen TV, you get two black columns on either side of the picture. These are called pillar boxes (the pillar box effect) or windowboxes.
Reference:
http://en.wikipedia.org/wiki/Pillar_box_(film)