In a Hybrid deployment, you will have several components as follows:

• On-premises servers: Exchange servers and AD Connect has requirements to implement Exchange hybrid
• Office 365: Your tenant where your Exchange Online is
• HCW: This is where you configure the connection between your environments

Learn more about Hybrid Configuration wizard at https://technet.microsoft.com/en-us/library/hh529921.

Learn more about Sharing and Collaboration at https://technet.microsoft.com/en-us/library/exchange-online-sharing-and-collaboration.aspx.

• Azure AD synchronization: Azure AD Connect synchronizes your on-premises directory to Azure AD to support a hybrid Exchange. It synchronizes several objects, such as users, distribution groups, security groups, and contacts. With this synchronization, you will have a unique Global Address List (GAL) available to the users. Keep in mind that when you have a hybrid Exchange environment, your on-premises objects are the source authority system. Azure AD Connect is a software component that is installed on an on-premises server separate to domain controllers and Exchange environments.

Learn more about Azure AD Connect at https://docs.microsoft.com/en-us/azure/active-directory/connect/active-directory-aadconnect-prerequisites.