Since many new members have asked this question I decided to make it a post. One of the primary concerns is that we are always told NEVER share your private keys. And this is true for Private keys that have access to your funds, HOWEVER, Steemit was designed to have multiple Keys for this very reason, to create different levels of access to your account based on which key you use.
First using your Owner Key, you can change your other keys (including your Posting key). This allows you to revoke access at any time provided you keep your Master Key safe.
Second your Active Key, is the one used to access your funds. Your posting keys can only be used to vote, author posts/comments, resteem and CLAIM rewards.
Posting Keys can not transfer funds or access any of the Steem, Steem Power or Steem Dollars your account has.
You can read in the Steemit FAQ (link below) on how each key is used and why sharing only your Posting Key does not place your funds at risk. (This is the same reason why if you login to steemit with your posting key you are asked for your memo or master key to transfer any funds).
To directly quote the Steemit FAQ: https://steemit.com/faq.html#What_are_my_different_keys_for
Posting key - The posting key allows accounts to post, comment, edit, vote, resteem, and follow or mute other accounts. Most users should be logging into Steemit every day with the posting key. You are more likely to have your password or key compromised the more you use it so a limited posting key exists to restrict the damage that a compromised account key would cause.
Remember NEVER share your Active Key unless you know what you are doing, and NEVER EVER EVER share your Owner Key as this is the one that can reset all other keys and ultimately steal your account and funds!
I hope this clears up any confusions new members have about their keys! If you have any other questions feel free to keep asking!
[Update: Fixed typos]