The unique feature of the system is that no promissory instruments are exchanged between the hawala brokers; the transaction takes place entirely on the honor system. As the system does not depend on the legal enforceability of claims, it can operate even in the absence of a legal and juridical environment. No records are produced of individual transactions; only a running tally of the amount owed one broker by the other is kept. Settlements of debts between hawala brokers can take a variety of forms, and need not take the form of direct cash transactions.Link (Thanks, Jens!)
In addition to commissions, hawala brokers often earn their profits through bypassing official exchange rates. Generally the funds enter the system in the source country's currency and leave the system in the recipient country's currency. As settlements often take place without any foreign exchange transactions, they can be made at other than official exchange rates.
Hawala is attractive to customers because it provides a fast and convenient transfer of funds, usually with a far lower commission than that charged by banks. Its advantages are most pronounced when the receiving country applies distortive exchange rate regulations (as has been the case for many typical receiving countries such as Pakistan or Egypt) or when the banking system in the receiving country is less complex (e.g. due to differences in legal environment in places such as Afghanistan, Yemen, Somalia).
Furthermore, the transfers are informal and not effectively regulated by governments, which is a major advantage to customers with tax, currency control, immigration, or other legal concerns. For the same reasons, governments do not favor the system, and accusations have been made in recent years that terrorist funding often changes hands through hawala networks.
I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.