Wallet Import Format (WIF, also known as Wallet Export Format) is a way of encoding a private ECDSA key so as to make it easier to copy. When users are demanded their WIF on the Steem blockchain, it refers to one of their private keys. For each user on Steem, there are 4 types of keys, each with their own public and private counterparts:
Everytime a user post,vote or transfer,he/she is signing each transaction with one of their private keys (depending on the action). Then, the system validates the transactions by using the public key. Authenticated transactions are added to the blockchain, while non-authenticated ones are rejected.
When a user's change their owner's key, all of their users keys (public and private) are changed.