In banking and finance, clearing denotes all activities from the time a commitment is made for a transaction until it is settled. Clearing of payments is necessary to turn the promise of payment (for example, in the form of a cheque or electronic payment request) into actual movement of money from one bank to another.
In trading, clearing is necessary because the speed of trades is much faster than the cycle time for completing the underlying transaction. It involves the management of post-trading, pre-settlement credit exposures to ensure that trades are settled in accordance with market rules, even if a buyer or seller should become insolvent prior to settlement. Processes included in clearing are reporting/monitoring, risk margining, netting of trades to single positions, tax handling, and failure handling.
Systemically important payment systems (SIPS) are payment systems which have the characteristic that a failure of these systems could potentially endanger the operation of the whole economy. In general, these are the major payment clearing or real-time gross settlement systems of individual countries, but in the case of Europe, there are certain pan-European payment systems. TARGET2 is a pan-European SIPS dealing with major inter-bank payments. STEP2, operated by the Euro Banking Association is a major pan-European clearing system for retail payments which has the potential to become a SIPS. The Federal Reserve System is a SIPS.
Source: WikiPedia