  Logical transductions provide a very useful tool to encode classes of
structures inside other classes of structures. In this paper we study
first-order (FO) transductions and the quasiorder they induce on infinite
classes of finite graphs. Surprisingly, this quasiorder is very complex, though
shaped by the locality properties of first-order logic. This contrasts with the
conjectured simplicity of the monadic second order (MSO) transduction
quasiorder.
  We first establish a local normal form for FO transductions, which is of
independent interest. Then we prove that the quotient partial order is a
bounded distributive join-semilattice, and that the subposet of \emph{additive}
classes is also a bounded distributive join-semilattice. The FO transduction
quasiorder has a great expressive power, and many well studied class properties
can be defined using it. We apply these structural properties to prove, among
other results, that FO transductions of the class of paths are exactly
perturbations of classes with bounded bandwidth, that the local variants of
monadic stability and monadic dependence are equivalent to their (standard)
non-local versions, and that the classes with pathwidth at most $k$, for $k\geq
1$ form a strict hierarchy in the FO transduction quasiorder.
