There are no stupid questions ;0)
I can’t remember if it goes when you create your account through SteemIt, but if you create your account through Blocktrades, you’re initially sent a password that allows you to log in. (I believe it is the same for SteemIt accounts).
This password is the key to everything in your account. With it, you’re in full controle. For example, if you want to change your password, you need that password.
Your private posting key can be found under the ‘Permissions’ tab in your wallet (your Master Password can’t). This key allows you to post, upvote, resteem and comment, but it won’t allow you to send transactions or edit your settings