Characterization of effective descent morphism

A faithfully flat morphism of commutative rings $A rightarrow B$ is an effective descent morphism. So is a regular monomorphism (right?). What is a characterization of effective descent morphisms?

Secondly, what are some reasons to work with the flat topology rather than the topology of effective descent morphisms? I am new to this, but is it not the case that faithfully flat is very similar to being an effective descent morphism?