Office 365 is a web-based subscription service that provides access to Microsoft Office tools from any device. It allows users to create and edit documents online or offline, collaborate with others in real-time, and access emails and calendars on any device. Key features include automatic online backup of files, built-in security protections, and 24/7 technical support. Users can access Office 365 tools and files through the Outlook web app or by installing the Office mobile apps on their devices.