Office 365 - Getting Started with Outlook on the web Presence and Instant Messaging

Instant messaging (IM) is a type of online chat which offers real-time text transmission over the internet.

Important: If you are a user of Skype for Business client, this client is being replaced by Microsoft Teams client - Office 365 - Getting Started with Teams.

Some of the benefits of using IM over other communication (email or phone) options:

You don’t have to open Skype for Business to have an IM conversation with people in your organization’s address book. You can start or accept an IM session with someone directly from Outlook on the web.

Microsoft provides extensive documentation on how to use this feature.