  The pourpose of these notes is to give an elementary description of one of
the simplest Berkovich's spaces: that associated to the projective line of a
complete and algebraically closed ultrametric field.
  \
  L'objectif de ces notes est de donner une description \'el\'ementaire de l'un
des espaces de Berkovich le plus simple : celui associ\'e \`a la droite
projective, o\`u le corps ultram\'etrique de base sera suppos\'e complet et
alg\'ebriquement clos.
