Horizon is a complete communications service for your business that provides an extensive range of fixed and mobile telephony capabilities via an easy-to-use web portal.
Office 365 is a cloud-based email, calendaring and collaboration service, hosted by Microsoft. It has inbuilt products to help provide collaboration between departments and employees. Including Exchange Online, OneDrive for Business and SharePoint.
Enterprise Resource Planning (ERP) could be describes as a database software package.