Allow users to send internal messages to each other. Settings> Users> Edit. Under "Teams", just tick "Enable user to send messages to other users"
Sending a Message
Click "Send Message" near the bottom on your left hand menus. Select the User from the drop down, type your message and send!
Receiving and Replying to a Message
Messages are shown on the receiving user's dashboard and you can click 'Send Reply' to quickly reply to the message. You can mark the message as read to remove it from your dashboard by clicking on the tick next to the message.
Send Message Associated to Lead
In a lead, click the '+' button at the top right of the lead summary, (next to the tabs), and choose 'Send Message'. The message you send will have a link to the lead summary and you can see which lead the message relates to.
You can use this feature to make other users aware of something happening on a lead where a task isn't really necessary. For example, if you take a call for a lead's assigned user and resolve the customer's query, you could send a message to make the assigned user aware. This situation doesn't need a task (nothing needs to be done), but you want to bring the other user's attention to it.