Communication is the heartbeat of Facebook and takes place in various forms, including three major ones:
"Status update" is what Facebook calls a message that you post via the publishing box that says "What's on your mind?" The publishing box (shown in the image above) appears at the top of both your home page and Timeline page. People use status updates to communicate their activities, post links to news stories, share photos and videos, and comment on life in general.
Messages are private notes you can send any friend you're connected with on Facebook; they're viewable only by the person to whom they're sent and do not go into the news feed or ticker for viewing by your network of friends. Rather, each message goes into the recipient's Facebook inbox which functions like a private email address. (Each user is actually assigned a email@example.com email address for this private inbox.) By default, messages also get forwarded to the external email address the user has provided to Facebook.
Chat is Facebook's name for its instant messaging system. You can engage in real-time conversation with any of your Facebook friends who happen to be online and signed in at the same time as you are. The Facebook Chat box is on the lower right side of the interface and contains a small green dot next to "Chat." Clicking it will open the chat box and show a green dot next to the name of friends who happen to be signed into Facebook at that time. Facebook Chat has a gear icon with settings you can change to determine who can see that you're online and when.