For a contract to use the Oraclize service, it needs to inherit the usingOraclize contract. You can find this contract at https://github.com/Oraclize/Ethereum-api.
The usingOraclize contract acts as the proxy for the OraclizeI and OraclizeAddrResolverI contracts. Actually, usingOraclize makes it easy to make calls to the OraclizeI and OraclizeAddrResolverI contracts, that is, it provides simpler APIs. You can also directly make calls to the OraclizeI and OraclizeAddrResolverI contracts if you feel comfortable. You can go through the source code of these contracts to find all the available APIs. We will only learn the most necessary ones.
Let's look at how to set proof type, set proof storage location, ...
