Carnival is an originally a pagan European spring festival,
with an emphasis on role-reversal and suspension of social norms. The feast was christianised by the Catholic Church and is celebrated in the three days preceding the Christian holidays of Ash Wednesday and Lent.
In secular society many people no longer associate the holiday with religion, but its date is still set before the start of the Christian season of Lent.
The core of the modern Carnaval has maintained its socially critical function by role reversal and temporary abandonment of social norms.
In many places, in particularly in the Southern Netherlands, large carnaval parades are held with large floats, organized and created by the carnaval associations. The parades have usually a particular theme whereby authorities are ridiculed and criticized, events of the past year are represented and satyrised to make people think outside the box or function as a mirror to society.
Groups or individuals on foot participate and fill the gaps between the carnaval wagons during the parade. Fanfares and marching bands provide for typical carnaval music. The floats are built by carnaval associations, but also often by independent groups of friends, families, neighbourhoods or other clubs.
(adapted from Wikipedia)