The International Confederation of Wizards (ICW) is a wizarding intergovernmental organisation, equivalent to the Muggle United Nations (JKR:Tw).
The leader of the International Confederation of Wizards is called the Supreme Mugwump. The first Supreme Mugwump was Pierre Bonaccord (OP31). Other holders of the office were Albus Dumbledore (PS4, OP15) and Babajide Akingbade (Pm).
The International Confederation of Wizards Educational Office maintains a worldwide register of Wizarding Schools (Pm).
The laws and regulations that cover the rating, concealment, monitoring and protection of magical beasts are devised and enforced by the ICW (FB).
The International Confederation of Wizards’ Quidditch Committee (ICWQC) organises and regulates the rules for the Quidditch World Cup tournaments (QA, Pm).
In 1692, the Confederation introduced the International Statute of Secrecy to hide the existence of magic from Muggle eyes (Pm, FB). This included the concealment of magical beasts. The right of wizards to carry a wand at all times was also established by the ICW in the same year (QA6).
Minister for Magic from 1718-1726, Damocles Rowle was censured by the ICW for his “tough on Muggles” policies (Pm).
In 1790, Rappaport’s Law was introduced by in the United States after a serious breach of the International Statute of Secrecy from within MACUSA. It required the complete segregation of wizards from the No-Maj community (Pm).
After attacks by an unknown force in New York City in 1926, the ICW told MACUSA President Seraphina Picquery that they might send a delegation to investigate because they believed the attacks were connected to outlaw wizard Gellert Grindelwald. They were correct (WFT).
Millicent Bagnold (Minister for Magic from 1980-1990), was called before the ICW following breaches of the International Statute of Secrecy after Lord Voldemort was vanquished by the baby Harry Potter. She was acquitted by acclamation (Pm).
Elsewhere in canon, we also hear mention of the International Confederation of Warlocks (CS2, OP2, OP8) and the International Federation of Warlocks (PA3). The most likely explanation is that JKR just got confused and misremembered the name she had chosen for this magical governmental authority, and that they are all intended to refer to the same governing body. Another plausible explanation could be that the International Confederation of Warlocks was the archaic name for this organisation and it subsequently changed its name to the International Confederation of Wizards. We know there was an International Warlock Convention in 1289 (CS9), so it's possible that the International Confederation of Warlocks was formed around that time, and later changed the Warlock part of its title to Wizards sometime before 1692 when the International Statute of Secrecy was passed by the ICW. This just leaves the lone mention of the International Federation of Warlocks, which was referred to in a Daily Prophet article concerning Sirius Black, so perhaps we can blame that on a typo by a harried journalist rushing to meet a deadline.
Lexicon list: International Government
From the Web
International Confederation of Wizards on Harry Potter Wiki