The 4 password Keys that Steemit uses
For all intents and purposes key and password mean the same thing (however Steemit sometimes uses the term password to refer to your owner key).
Keys.png
The Permissions page.
Steem/Steemit has 4 different key types:
Posting Key - this is what you use to make posts, comment and upvote/flag using your account. This is what you should use most of the time.
Active Key - this allows you to interact with your wallet i.e. sending Steem/SBD etc. It also lets you change the information on your profile. Only use it when you absolutely have to (i.e. to move Steem/SBD around) and keep it offline the rest of the time.
Owner Key - this is your master password. It lets you do everything, including changing to a new owner key, which will change all your other keys. You should only use it for this purpose and keep it offline at other times. You can change this by using the "password" page. Do not lose it.
Memo Key - this doesn't really do anything right now but will be used to sign messages (i.e. confirm you sent them) at a future date. You don't need to use this at the present time.