Let's look at the process by which an Ethereum smart contract can fetch data from other blockchains and the World Wide Web using Oraclize.

To fetch data that exists outside of the Ethereum blockchain, an Ethereum smart contract needs to send a query to Oraclize, mentioning the data source (representing where to fetch the data from) and the input for the data source (representing what to fetch).

Sending a query to Oraclize Oraclize means sending a contract call (that is, an internal transaction) to the Oraclize contract present in the Ethereum blockchain.

The Oraclize server keeps looking for new incoming queries to its smart contract. Whenever it sees a new query, it fetches the result and sends it back to your contract by calling the _callback method of your contract.