the ISF is a procedure needed to clear US customs in case of ocean shipments.

Since 2009 the US customs require the importer (the customer) to fill in an "Importer security file". This is applicable only to ocean freight (so shipments by air do not require it.)

The importer security for is quite bureaucratic and time consuming.
You have to submit before the shipment leaves the port of origin otherwise there is the risk (theoretical) that your shipment may be destroyed and you may be billed for the disposal of it...
But there is no pitfall, and the staff from our forwarder will help you.
Please find more information here:

http://www.cbp.gov/sites/default/files/documents/10_2faq_0.pdf