Why does cricket have Test, ODI and T20 formats?
Because the sport kept adding shorter versions without abandoning the long one, and each format now serves a different audience.
Test cricket is the original form and the longest. Matches last up to five days, each side normally batting twice, with no limit on the number of overs. A draw is a common and legitimate result if time runs out before either side wins, which is unique among major sports. Tests are played in white clothing with a red ball, and are widely regarded as the format that most tests technique and temperament.
One Day Internationals appeared almost by accident. In 1971 a washed-out Test in Melbourne was replaced with a limited-overs exhibition, which drew a large crowd. ODIs settled at 50 overs per side, are completed in a day, use a white ball and coloured clothing, and always produce a result barring weather.
Twenty20 was introduced in England in 2003 to attract audiences who would not commit a day. Each side faces 20 overs and matches finish in about three hours.
The formats reward different skills. Test cricket rewards patience and the ability to bat for hours; T20 rewards immediate scoring and specialist death bowling. Some players excel at one and never adapt to another.
They coexist commercially rather than comfortably. T20 leagues generate most of the sport's money and increasingly compete with international fixtures for players' availability, which is a live tension in the game's administration.
A fourth format, The Hundred, uses 100 balls per side. Domestic first-class cricket is the multi-day format below Test level.