WIF - Wallet Import Format
WIF is a Base58 format for private ECDSA keys for easier storing/copying. The acronym
wif is used in many steem libs as a parameter name for a key.
More technical documentation here.
Steem supports 4 different keys for the interaction with the network:
Owner: Thats the master key to control everything for a user. It can change all other keys including itself.
Active: Transaction key for spending Steem, powering up/down etc. Cannot change owner key.
Posting: Social actions, only for posting and curation.
Memo: Encrypt and decrypt private messages/memos e.g. in transactions.