A history of tennis
Ballgames are nearly as old as mankind. In 11th century France a game was played that was very similar to current tennis: jeu de paume. It was a game of returning a ball with the hand. Originally it was played in religious circles, but soon it became hugely popular in high society. We know for instance that king Louis X died of a cold, which he picked up during a game of jeu de paume in the Bois de Vincennes.
Power to the people
Jeu de paume then spread over high class people in the whole of Western Europe. After that the whole of French, Italian, British, Spanish and German societies embraced the sports, which was very much to the dislike of the upper class. They tried to monopolize the sports, but they failed. In England, religious services were often disturbed because people used church walls to bounce the balls during their play. Even worse was that the players used far from civilized language to express their anger for a missed ball.
As said, in the beginning only hands were used. But increasingly, people started to use equipment to control the ball. First a glove, followed by a bat (as used in baseball or cricket), which eventually evolved into a racket. This was around 1500. This jeu de paume racket (the word comes from the Arab word for the palm of your hand) was the direct predecessor of all modern rackets used in tennis, badminton, squash, etc.
Around 1600, France was addicted to the racket sports. It was played in open field, but also inside tennis courts were built in cities, castles and universities. French capital Paris counted around 250 inside courts in this period. When theatre got popular after 1600 however, jeu de paume was pushed to the background. Court owners found out that financially they gained more by exploiting their tennis accommodations as theatres.
Although I have been talking about jeu de paume mainly, the earliest notion of tennis stems from 1399 already. It was then spelt as tenez, which probably is the origin of the word tennis (it means "here it comes" in French - called out by the serving player). Other theories however say that the word was derived from the ancient Egyptian city Tinnis, well-known for its linnen which was used to fill the balls.
The official tennis game was born almost 500 years after this. The British major Walter Wingfield designed the basic idea for the modern tennis game. In 1873 he patented his idea, which he called Sphairstikè, soon nicknamed 'sticky'. The counting of the points was exactly the same as used in jeu de paume.
The way points are counted in tennis is mysterious. It is probably based on a Middle Age math system. According to one theory, the points were counted by using a clockwork. In each game a player had to gain sixty points by winning four hits. Each won rally is fifteen points (for instance 15-0, 15-15, 30-15, 45-15, game). Another theory claims that the early players gambled: one point was worth 15 centimes. All in all they are all incertain guesses, and none of them gives an answer to the question why the points go from 30 to 40, instead to 45, in a game.
Love and deuce
Thinkers have also theorized about some other curious tennis terms, like 'love' and 'deuce'. It is said that the shape of an egg (l'oeuf in French, sounds pretty much the same as love in English) was to symbolize the zero, while 'deuce' is deductable to 'deux' or 'deux à jouer': two points left to play.
Differently shaped courts
Wingfield's invention immediately attracted lots of cricket and crocket clubs, although his rules were criticized. Each club interpreted these in its own way, which caused differently shaped courts and nets everywhere. In 1877 the famous tournament of Wimbledon was played for the first time at the lawns of the All England Crocket Club. Three years later the Marylebone Cricket Club set strict rules for courts and rules for tennis. Of the original Sphairstikè, a lot of elements had gone and Wingfield became bitter over the fact that only after many changes his game had gotten popular. The 1880 rules are still used, although they have been bent in some ways, for instance with the introduction of the tie-break.
With the standardized rules, tennis started a true conquering tour around the world. Most important exponents here were the United States (introducing the overhand service) and Australia (introducing the smash). In the beginning tennis was only practised outside on grass, but soon gravel was used as well. Later experimenting lead to matches on concrete, hard-court and other surfaces.