Composantes connexes et irr\'eductibles en familles

For an algebraic stack $\sX$ flat and of finite presentation over a scheme $S$, we introduce various notions of {\em relative connected components} and {\em relative irreducible components}. The main distinction between these notions is whether we require the total space of a relative component to be open or closed in $\sX$. We study the representability of the associated functors of relative components, and give an application to the moduli stack of curves of genus $g$ admitting an action of a fixed finite group $G$.