SQL Centre - a centralised repository on all things SQL
SQL Server
Microsoft SQL Server is a relational database server, originally based on Sybase SQL Server and ported to OS/2 and Windows platforms. Microsoft currently support SQL Server 2008 R2, SQL Server 2008 and SQL Server 2005.
Available only on Windows platforms, SQL Server there are a number of different editions of SQL Server ranging from the free SQL Server Express aimed at competing with MySQL, Enterprise aimed at competing with Oracle, and DataCenter. There are also specialised editions like SQL Azure (cloud-based) and SQL CE (Compact Edition).
MS Access
Microsoft Access is a desktop database as opposed to server-based like Oracle, SQL Server and MySQL. The major difference is that it isn't just a database - it is a rapid application development tool that includes everything you need to build the front-end of the application as well as the back-end database. However, it is possible to scale the application by splitting the database off into a separate file - or using a different database management system altogether.
Oracle
Oracle is regarded as the first commercial relational database management system, with its origins dating back to the late 1970s. Although designed for mainframes and unix for enterprise-scale applications, Oracle is now available on the Windows platform and has often led the way in terms of new database features, pioneering triggers, stored procedures and a programming language around SQL.
MySQL
MySQL is an open source relational database management system that has enjoyed fantastic success powering web-based applications such as Joomla, Drupal, WordPress and Moodle.