What are chat rooms?
Each chat room defines a specific topic; the conversation in that room is expected to loosely follow the topic. Chat typically has a few established rooms for common topics.
When should I create a new chat room?
When you find yourself wanting to have an extended conversation about a topic that is not covered by one of the existing chat rooms, you should create a new room for it.
How do I create a new chat room?
There is a link to chat in the footer of every page. The link also appears in the Stack Exchange site switcher on the upper right corner of every page. Click one of those links, and then click the "create a new room" button at the bottom of the Rooms page.
What happens when I create a new room?
After you create the room, you'll want to invite other users into your room -- either using the invite room menu option, or by pasting the URL to your room as a chat message in other rooms you're participating in.
You can share the chat room URL with the broader internet, but bear in mind that only users who hold the chat privilege on the parent site will be able to talk in your room!
What other new privileges in chat do I get?
At 100 reputation you also get access to the "Upload image" button in chat.