Both partners and IBs are considered partner types, but refer to differing levels of partnership. Everyone starts as a trading partner and becomes an IB once certain criteria are met.
IBs not only have higher requirements than trading partners, but they are also rewarded a greater share of commission. Other than the specific benefits of being an IB, there is little functional difference between partners and IBs. You can also lose IB status, but you can never lose partner status.
Follow the link to find out how to become an IB.