SQL Server is a relational database management system developed by Microsoft that supports the SQL language. It has several editions with different capabilities and limitations. The SQL Server architecture consists of three main layers - the SQL Server Network Interface, Database Engine, and SQLOS API. The Database Engine handles query processing and storage. SQL Web and Express editions have limitations such as restricted CPU cores and database size. SQL Azure also has limitations compared to on-premises SQL Server editions such as different T-SQL syntax and inability to switch databases.