Microsoft Internet Security and Acceleration Server 2006 (ISA Server 2006) is an integrated web proxy and firewall product, commonly used for outbound Internet access and securely publishing intranet websites to the Internet.
ISA Server 2006 is 32-bit only, and runs only on Windows 2000 Server and Windows Server 2003.
The 64-bit version of ISA Server is called Forefront Threat Management Gateway, or TMG, and runs only on Windows 2008 and Windows 2008 R2, x64 editions.
ISA Server supports authentication delegation, and can pre-authenticate incoming requests before sending them to the target web server. It can also use Kerberos credential delegation (KCD) or protocol transition in order to present credentials securely to published web servers.
It is commonly used as an outbound web proxy, due to its integration with Active Directory, and support for transparent user authentication via Windows Integrated (Negotiate; Kerberos or NTLM).
The Enterprise Edition supports servers being grouped into Arrays, which share common firewall policy and configuration settings, and integrates with Windows Network Load Balancing (NLB) to provide failover clustering.