Samba is the most famous of the various forms of music arising from African roots in Brazil. The name samba most probably comes from the Angolan semba (mesemba) - a religious rhythm. Samba developed as a distinctive kind of music at the beginning of the 20th century in Rio de Janeiro, then the capital of Brazil.

Originally the Samba sound evolved under the strong influence of former african slaves who had migrated from Bahia, a state in the north-east of Brazil, to an area of Rio known as "little Africa". Later, property speculation drove the first Sambistas out to the favelas where the musical style was popularised until it became synonymous with Brazil.

Samba has gathered influence from many different musical styles and can involve a wide range of instruments including the voice. The music is most identified with the drum orchestra (bateria) as heard in the Rio Carnivals. The Sheffield Samba Band is a drum orchestra and currently play:

• Surdo (large bass drums) pound out the heartbeat, their incessant drive lays foundation for the multiple cross rhythms of the bateria,
• Repenique is a midsize drum often played by the maestro (director of the orchestra),
  
• Caixa (snare drum) rattle away in hypnotic frenzy,
• Tamborim or Tam (tambourines without cymbals) hit with special sticks carry a high-pitched rhythmic phrasing,
• Agogõ (double cowbell) accent with metallic tones,
• Afoxe or xequre is played by moving beads around a hollow gourd, its place can also be taken by a more standard shaker known as a gansa or chocalho.

West African rhythms such as Samba are usually polyrhythmic (multiple patterns interweaved) and seek the occurrence of at least two different rhythms at the same time. It is the juxtaposition of opposing rhythms which creates the vital spark.

This information came from:
Wikipedia
All Brazilian Music samba page
Carnaval guide to Rio