  We introduce the notion of {\it approximation type} for the partial, and in
certain cases the total description of extensions of a given valuation from a
field $K$ to the rational function field $K(x)$. To every extension, a unique
approximation type of $x$ over $K$ is associated, while $x$ may be the limit of
many pseudo Cauchy sequences. Approximation types also provide information in
cases where the extensions are not immediate, and we prove that they correspond
bijectively to the extensions when $K$ is algebraically closed or lies dense in
its algebraic closure.
