Quote from LeeD:
Well, a broker (the agent that executes trades) and the clearing firm (which does paperwork regarding stettlement and holds deposit for overnight leverage) don't have to be the same company. In fact, a number of clients use IB as a broker (because of access to wide range of markets) but clear through a different firm (there are firms that offer better interest rates and leverage than IB).
If you are an exchange member (remember membership can be leased), you can execute trades in the exchange directly. This set-up originated in floor trading where exchange members were on the floor and traded against each other and anyone else wishing to execute a trade had to call up a floor trader.
If you are trading only your own account (as a person or a firm) you don't need to register as futures commission murchant etc.
An alternative to exchange membership is "unfitered access". In this case orders are still sent via the broker but they bypass checks necessary to establish if the trader has sufficient funds to cover whatever margin is required. This is possible because exchanges often set requiremnts on the overnight margin only and the intraday margin is up to the broker (and can be zero). Bypassing account checks makes substantial difference in speed of access.