Basically as xandman said. Corporate membership is similar to individual membership but it applies to your bonafide employees. Clearing members interface with the exchange directly, have some additional regulatory issues and have the ability to pass on / guarantee their customers trades. So, when CHF goes nuts the CME looks to the clearing members ( like IB) to make good on the trades. Then it's IBs resposability to go after its customers individually. They only get member rates themselves if they become corporate members and are trading their own account.
This whole business plays out like a Michael Lewis book. It's hard to believe it if you haven't lived through it.