The informal century
For much of chess history, there was no official way to become World Champion. The title was informal and loose. The turning point usually cited is 1886, when Wilhelm Steinitz played a match against Johannes Zukertort in the United States. At Steinitz's insistence, the contract described the contest as being "for the Championship of the World," making it the first match ever set up in those terms. Steinitz won and is generally recognized as the first World Champion. Some historians push the start of his reign back to 1866, when he beat Adolf Anderssen, which is why you will sometimes see different dates. Either way, the pattern was set: the title was settled by a match between two strong players, arranged privately.
FIDE takes charge
In 1924 the Fédération Internationale des Échecs, known as FIDE, was founded in Paris to govern chess internationally. Over the decades it took control of the World Championship and built a formal system around it. Today FIDE's most visible activity is running the World Championship cycle, and winning it is governed by written regulations rather than private agreements.
The modern cycle
The modern structure works like this. The world title belongs to the reigning champion, who defends it against a challenger. That challenger earns the right by winning the Candidates Tournament, a knockout competition held every cycle that draws the strongest players not holding the title.
The Championship itself is a match, not a single game or a tournament. The champion and challenger play a scheduled series of games, typically twelve for the classical title. Each game counts as a point, worth a half for a draw. The first player to reach a set total, such as 6.5 points, wins the match and the title. If the series is level after the scheduled games, the rules provide tie-break games, played with shorter time limits, and eventually tie-breaking procedures to force a result.
The 2013 match shows the system in action. Magnus Carlsen won the Candidates Tournament in London, then challenged the reigning champion Viswanathan Anand in Chennai, India. They played ten of the twelve scheduled games before Carlsen had scored 6.5 points, ending the match and making him World Champion at 22.
What changed from the old days
The biggest shift is predictability. In the nineteenth century, who had a claim to the title was genuinely murky, with rival champions and disputed results. Today FIDE runs a single, rule-based cycle: the Candidates Tournament picks the challenger, the champion defends in a match with agreed game counts and tie-breaks, and the winner is declared for that cycle. The romantic age of two gentlemen simply agreeing to settle it on the board is long over, replaced by governed events with schedules, arbiters, and prize funds. For all the formality, though, the heart of it is unchanged: one night, two players, and whoever plays better across a hard-fought match wears the crown.