From 2014 to 2016

SQL Server 2014 introduced the In-Memory OLTP feature.

AlwaysOn feature has been enhanched in SQL Server 2014 by having the maximum number of secondary replicas increased to 8 and allowing to use Azure as a Replica for an AG.

Column Store Indexes are now updateable in SQL Server 2014.

Azure storage is now available for storing database files.

Controling the Disk I/O with Resource Governor is now possible with SQL Server 2014 as it is now also possible to use SSD as Buffer Pool Extension. Backup Encryption and Incremental Statistics are also two new features in SQL Server 2014.

From 2012 to 2014

SQL Server 2012 introduced a new High Availability solution called AlwaysOn. It also introduced the Contained Databases and the Column Store Indexes.

With SQL Server 2012 it's now possible to create user defined Server Roles and Auditing at Database level.

SQL Server 2012 supports now up to 15,000 partitions by default and it also allows back up directly to and restore from the Windows Azure Blob storage service with a maximum of 1TB size.

New and Enhanced Spatial Features, Ad-hoc Query Paging Implementation, Sequence Objects and the SQL Server Express LocalDB are some of the new features that will help developers.

SQL Server 2012 also delivers new tools like the Distributed Replay Controller and the Data Quality Services (DQS) a new BI tool that complements MDS.

From 2010 to 2012

Based on SQL Server 2008 but with an extensive set of new features.

SQL Server 2008 R2 Datacenter Edition has been added to the top of the relational database product lineup and brings the SQL Server product editions in-line with the Windows Server product editions, including its Datacenter Edition. SQL Server 2008 R2 Datacenter Edition provides support for systems with up to 256 processor cores. In addition, it offers multiserver management and a new event-processing technology called StreamInsight

The other new edition of SQL Server 2008 R2 is the Parallel Data Warehouse. The PDW Edition is an appliance available only through select OEMs such as HP, Dell, and IBM and is integrated with SSIS, SSAS, SSRS.

SQL Server 2008 R2 includes a new Utility Explorer, which is part of SSMS, and lets you create a SQL Server Utility Control Point where you can enlist multiple SQL Server instances to be managed up to a maximum of 25 SQL Server instances.

Master Data Services is a new feature that gives the ability to create a master data definition for the enterprise to map and convert data from all the different date sources into that central data repository.

From 2008 to 2010

SQL 2008 introduced a new feature called "Central Management Servers" (CMS) which allows the administration of multiple SQL Server instances. With CMS is now possible to run a query or policy over multiple SQL Server instances simultaneously.

Introduced in SQL Server 2008, Policy-Based Management allows define and enforce policies for configuring and managing SQL Server instances.

Other new feature is the Performance Data Collection and Warehouse that uses data collectors to gather performance data from multiple SQL Servers and store it within a Management Data Warehouse (MDW).

With SQL Server 2008, Data Compressions is supported by row and page compression for both tables and indexes. Native Backup Compression it is also now supported.
SQL Server Resource Governor is a new feature to manage SQL Server workload and system resource consumption by specifying limits on the amount of CPU, physical IO, and memory that applications or users can use.

In the Security area SQL Server 2008 offers Extensible Key Management (EKM) and Transparent Data Encryption (TDE). TDE performs real-time I/O encryption and decryption of the data and log files and EKM enables the encryption keys that protect the database files to be stored in an off-box device.

SQL Server 2008 introduced new data types as the GEOMETRY and GEOGRAPHY for storing spatial data and FILESTREAM to store unstructured data, such as documents and images, on the file system.
Change Data Capture (CDC) is a new feature in SQL Server 2008 for data tracking. It records insert, update and delete activity in SQL Server tables.

From 2005 to 2008

Included native support for managing XML data (it defined a xml data type). This version also introduced Common Language Runtime (CLR) integrated with the .NET Framework. Native support for Partitions on tables and indexes, so scaling out a database onto a cluster became easier.

SQL Server 2005 introduced DMVs (Dynamic Management Views), which are specialized views and functions that return server state information that can be used to monitor the health of a server instance, diagnose problems, and tune performance.
SQL Server 2005 also introduced Database Mirroring that was evaluated with the SP1.

Page checksum is new feature in SQL2005 that provides you a stronger mechanism than torn-page to detect any corruptions in IO path.

SQL Server 2005 introduced two new isolation levels: SNAPSHOT and READ COMMITTED SNAPSHOT.

Notification Services, Service Broker, Integration Services and Tools (SSMS, SSCM).

From 2000 to 2005

More improvements for the DBA including an index tuning wizard which suggests potential indexes to be placed on tables. Introduction of user defined functions and partitioned views and functionality to support XML. Log Shipping and Multi SQL Server instances support has also new features in SQL Server 2000.

The OLAP Services feature available in SQL Server version 7.0 is now called SQL Server 2000 Analysis Services.

In 2003, Microsoft released SQL Server 2000 64bit version that had the code name "Liberty".

From 1998 to 1999

SQL Server 7.0 included modifications and extensions to the Sybase code base, adding support for the IA-64 architecture.

Microsoft re-engineered the product to use native Windows NT files for a more logical integration with backup systems. Many of the configuration parameters became ‘self-tuning’ to avoid the need for a DBA on smaller systems.

In 1999 was released the OLAP tools for SQL Server 7.0.

1995

SQL Server 6.0 was the first version designed for NT, and did not include any direction from Sybase, since Sybase and Microsoft parted ways and each pursued its own design and marketing schemes. Microsoft negotiated exclusive rights to all versions of SQL Server written for Microsoft operating systems.
SQL Server 6.0 is a rewrite of the original SQL Server product that takes advantage of the Windows NT Operating System and allows remote management of a collection of enterprise wide servers. Microsoft have incorporated advanced features such as Replication and support for multi-processor hardware in this version.

1996
Microsoft released Microsoft SQL Server 6.5 version that contains several performance improvements particularly in various contention scenarios when many users are attempting to add records and compete to add sequentially to a clustered index for example.

Replication is also much improved and can now replicate with other ODBC data sources as well as interface with Oracle or other more complicated corporate situations.

From 1992 to 1993

Already without Ashton-Tate, that was been sold to Borland in 1991, Microsoft and Sybase developed SQL Server 4.2 and shipped it in 1992, bundled with OS/2 version 1.3, followed by version 4.21, the first version for Windows NT and was released alongside Windows NT 3.1

From 1989 to 1991
This was the first version of Microsoft SQL Server.  Ashton-Tate (company that developed the popular dBase), Microsoft and Sybase worked together on the release of SQL Server for IBM OS/2, a 16bit variant version of Sybase and served as Microsoft's entry to the enterprise-level database market.

In 1991 SQL Server 1.1 was released.