SQL Server can be hosted entirely in Microsoft Azure, either in a hosted virtual machine (VM) or as a hosted service. Hosting a virtual machine in Azure is known as infrastructure as a service (IaaS), and hosting a service in Azure is known as platform as a service (PaaS). Microsoft’s hosted version of SQL Server is known as Azure SQL Database or just SQL Database that is optimized for software as a service (SaaS) app development.
SQL Server also can be deployed in a hybrid cloud scenario, extending your on-premises SQL Server environment to utilize various features of the Azure platform:
SQL Server backup to URL: You can back up your database directly to Azure blob storage or back it up to an on-premises file store and then copy it to Azure blob storage. Using this option can save precious storage space on expensive local storage.
SQL Server data files in Azure: You can use Azure blob storage for database files for an on-premises instance of SQL Server. Although this option primarily is used with Azure virtual machines, it has its place when developing and testing functionality in some scenarios.
Stretch SQL Server table to Azure SQL Database: You can stretch an on-premises table to store cold and warm data (older data) in Azure SQL Database while hot or current data (more recent data) remains in the on-premises table—with all of the data being available to query. This option became available in SQL Server 2016 and is a great way to archive infrequently accessed data off your local storage. This option can save money and increase performance for online transactional processing (OLTP) operations on hot data, while the data remains available for analytical queries.
Transactional replication to Azure SQL Database: You can use transactional replication to replicate data from an on-premises or IaaS SQL Server database to Azure SQL Database. This option is useful to replicate data close to different groups of users to improve query performance and as a prelude to migrating to Azure, enabling you to minimize downtime during migration.
AlwaysOn Availability Group replica in IaaS: You can configure SQL Server in IaaS as an asynchronous replica of an AlwaysOn Availability Group. This option provides you with a low-cost disaster recovery scenario and can be used as a prelude to migrating to Azure, allowing you to minimize downtime.
Finally, Microsoft offers an additional PaaS service using SQL Server for data warehouse solutions, called Azure SQL Data Warehouse. Azure SQL Data Warehouse is an enterprise-class, distributed database capable of processing massive volumes of relational and non-relational data.
Source of Information : Migrating SQL Server Databases to Azure