Factorization semigroups and irreducible components of Hurwitz space

We introduce a natural structure of a semigroup (isomorphic to a factorization semigroup of the unity in the symmetric group) on the set of irreducible components of Hurwitz space of marked degree $d$ coverings of $\mathbb P^1$ of fixed ramification types. It is proved that this semigroup is finitely presented. The problem when collections of ramification types define uniquely the corresponding irreducible components of the Hurwitz space is investigated. In particular, the set of irreducible components of the Hurwitz space of three-sheeted coverings of the projective line is completely described.