For Steemit you have 4 different keys with both public and private versions. If you click on the Permissions tab there is an explanation of the role each key serves which I have copied here for your convenience:
Posting: The posting key is used for posting and voting. It should be different from the active and owner keys.
Active: The active key is used to make transfers and place orders in the internal market.
Owner: The owner key is the master key for the account and is required to change the other keys. The private key or password for the owner key should be kept offline as much as possible.
Memo: The memo key is used to create and read memos.
As stated next to the Owner key for maximum security you should only use it when you have to. Therefore if you are logging in just to post then you should use your posting key only to do that. This ensures that if it is stolen or hacked the person who takes it can only post on your account and not actually make any transfers or steal from you.