The Les Houches Accords are agreements between particle physicists to standardize the interface between the matrix element programs and the event generators used to calculate different quantities.
The original accord was initially formed in 2001, at a conference in Les Houches, in the French Alps, before it was subsequently expanded.
Depending on specific properties of the particle decay that physicists are interested in, they may desire to use a certain program for these tasks, but before the Les Houches Accords, there was no general interface for communicating between the programs.
The Accords also make it easier to generate parton distribution functions, which are datasets used to calculate cross sections, for events.
The original Accord defined a programmatic interface for transfer of event information, in terms of Fortran common blocks, but no data exchange file format was defined until 2006.