Each asset supported by Iron Bank is integrated through a iToken contract, which is an EIP-20 compliant representation of balances supplied to the protocol. By minting iTokens, users 

(1) earn interest through the iTokens exchange rate, which increases in value relative to the underlying asset, and (2) gain the ability to use iTokens as collateral.

iTokens are the primary means of interacting with Iron Bank; when user mints, redeems, borrows, repays a borrow, liquidates a borrow, or transfers iTokens, they will do so using the iToken contract.
There are currently two types of iTokens: CErc20 and CEther. Though both types expose the EIP-20 interface, CErc20 wraps an underlying ERC-20 asset, while CEther simply wraps Ether itself.