Watch the video demo here (Flash Video)
Moodle has a built-in messaging system. You can use this system to contact your instructor or your classmates and they can contact you in this same way. Using the Messaging feature of Moodle requires that you have pop-ups enabled for the class site. If you are in your class site when someone sends you a message the message will pop-up on your screen. If you are offline when a message is sent to you, it will get copied and sent as an email to the email account that is set up in Moodle. (Note: You can change both of these default behaviors in your Moodle profile.)
You can send a message in a number of different ways. The easiest way to send a message is to click on the little envelope icon next to the person's name in the Online Users block (Note: The Online Users block is optional so you may not see it in your class site.)

Another popular way to send a message is to click on Participants in the People block. This will take you to a page showing the names of all of the people in your class. Click on the name of the person that you want to send a message to, and then click on the Send message button. This will pop open a new window where you can type your message. Then click the Send message button on this window to have the message sent to the participant.
As mentioned earlier, if you're online when someone sends you a message a pop-up will alert you to an incoming message. It will show your list of contacts, with online contacts at the top and offline contacts at the bottom. The contact sending you a message will be indicated in bold, and the number after the contact's name will indicate the number of waiting messages from that person. To retrieve the message, click on the name of the person and you will see your message. You'll be able to reply to the message on that same window. If you are offline when a message is sent to you, you will receive notification of new messages the next time you login to the class. A copy will also be sent to your email after a certain time frame, which you can customize by clicking on Edit Profile>>Messages>>Settings.
While it is fairly easy to locate your friends or classmates and send them
messages, it is more convenient to have your contacts stored in your messaging
window. You can add contacts regardless of whether or not they are online. To
add an online contact while sending them a message or reading a message from
them, click on the Add contact button
from within
the messaging interface. Once added, contacts will appear each time you access
the messaging interface.
To block someone from being able to send you messages, click on the Block
contact button
. You
never want to block your instructor for obvious reasons.
If you are not actively sending a message, you can add a contact through the
search feature. To locate a contact, click on the Search tab in the
Messages area. Type in your contact's information in the box, then press
Enter or click on Search. Use the same icons as above to add contacts
to your contact list.