Keys are much important here. Just remember 'Private posting key' is required to login into steemit.com.

The 'master password key' is the most important and never share it with anyone until you trust him/them. With this, you can log in to steemitwallet.com.

'Private Active Key' is required when you want to transfer tokens. This can be done with the master password key too.

'Private Owner Key' is another important thing because without this you can't change the master password key if it gets exposed. Once the master password key is changed all other keys automatically get reset. That means previous posting, active and owner keys will not work anymore.

Memo Key and Public key are not important.

Thanks to @greenman & @offgridlife