You are already familiar with private keys on Steem by now. But here is short analogy to remember what key stands for, what and how you should secure yourself.
Posting private key
This key is for your house, where anyone who has access to your keys can go inside your house, use your common stuff on behalf of you because you have some level of trust to them. In a case of Steem, Voting, Commenting, Posting, Reputation
Active private key
This key is for your safe/vault where you keep your money, not many people know passcode but only you. Who has access to this key can control anything that is inside that safe, plus your identity. In a case of Steem, Control your balances and funds, account details
Owner private key
This key is your key to your house and safe/vault, as well as give access or grant permission to own house entirely. In a case of Steem, Ownership of account
Memo private key
This key is your signature where you sign document or important messages/mails. In a case of Steem, Encrypt messages, private communication.
You can share any of the above permissions with other accounts on Steem.
Just like, you can rent out room in your house or have more than one passcode to access safe/vault or joint ownership of property/house or appoint attorney.
Conclusion
Always know how your private keys are being used and who has access to them. Use them with caution!
Better be safe than sorry or to make it analogy related, Be cautious, don't blame neighbors! :)