FACTBOX: Brazil's Carnival - Let the fun begin!

(Reuters) - Brazil's Carnival is a riot of drinking, dancing and partying, symbolized worldwide by the nearly naked Carnival queens who strut their stuff in the Samba parades.

Carnival in Rio de Janeiro gets going Friday when the mayor gives the keys to the city to King Momo, the Lord of Misrule. It will run through Ash Wednesday.

* HISTORY: Rooted in ancient festivals, Brazil's modern Carnival dates back to the 18th century and mixed the traditions of Portuguese settlers with the African culture of the slave population. The first parade of Samba schools was in 1928. Carnival is supposed to usher in a 40-day fasting period before Lent.

* SAMBA SCHOOLS: The top 12 Samba organizations, known as schools, parade in Rio's Sambadrome stadium over Sunday and Monday nights to compete for the champion's crown. Each school features up to 6,000 drummers, dancers and other participants as well as spectacularly decorated floats. The schools develop an allegorical theme, or enredo, with a specially written Samba song. Beija-Flor (Humming Bird) is the reigning champion.

* RAINHAS: Each school has a rainha, or queen, who leads the drums corps. Usually dressed in little more than a huge plumed headdress and high-heeled shoes, their role has become controversial as famous actresses or models now usually fill the role instead of someone from the school.

* BLOCOS AND BANDAS: Bands and social groups that take part in street parades are an important part of keeping traditional Carnival alive as critics say the Sambadrome parades, where the best seats can cost up to $1,000, have taken it away from the people's reach.

* WHERE THEY ARE: Carnival in Salvador, in northeastern Brazil, now competes with Rio for popularity with much of the action taking place in the streets where "trio electric" -- musical combos on the back of trucks -- play to the crowd. The historic town of Olinda, near Recife, also has a noted Carnival.

Celebrations also take place in the U.S. city of New Orleans, across Latin America and the Caribbean, with Trinidad's being especially colorful.

(Reporting by Stuart Grudgings; Editing by Kieran Murray)