Hi, Marcin. It's very simply - on before triggers you make actions
that should be done before data is changed, on after - that should or
can be done after it. About delete items - certainly on before,
because another will break references integrity. On before insert
triggers people usually check some conditions or calculate some
columns or set defaults and other stuff of this kind. On after
triggers it is reasonable to support logs, stored aggregates and so on
- if action failed, server don't make unneeded work.

Best regards.