 Fiber of the map on spectra

The fiber over $\mathfrak{p}\in \mathrm{Spec}A$ of $\mathrm{Spec}B\to \mathrm{Spec}A$ is the set of primes $\mathfrak{q}\subset B$ satisfying $\mathfrak{q}\cap A=\mathfrak{p}$. It is naturally the spectrum of the fiber ring $B{\otimes }_A\kappa (\mathfrak{p})$.