What are the differences between ACH, SEPA, and RTGS payment systems?

ACH is usually referred to as the US (e-check) system that takes between 3-4 days to clear. All ACH payments in the US are done by one of the two ACH: either the EPN or FedACH. ACH rules are governed by NACHA.

RTGS is a settlement technique, it is used by various central banks around the world to do large payment settlement of their member banks. Best way to describe RTGS is as a real-time settlement system for banks for large value payments. Banks hold Nostro accounts with the central banks and all large payments are settled on a gross basis by the central bank. It is also the mechanism via which deferred net settlement is done on a periodic basis.

SEPA is the European equivalent of bank transfer for all countries Euro. It is implemented in all the countries where Euro is and a couple of other participating countries. So the payment is a single payment instruction/set regardless if the payment is being made within the Euro country or to another (cross-border) Euro country. It is not real-time yet, but eventually would be heading there.