The Cisco ASA provides advanced stateful firewall and VPN concentrator functionality. It has long been the industry standard for firewalls. For more information on this product, see http://www.cisco.com/go/asa.
Firepower Threat Defense provides unified next-generation firewall and next-generation IPS functionality. In addition to the IPS features available on Firepower Software models, firewall and platform features include Site-to-Site VPN, robust routing, NAT, clustering, and other optimizations in application visibility and access control. Firepower Threat Defense also supports Advanced Malware Protection (AMP) and URL filtering. For more information on this product, see http://www.cisco.com/go/ngfw.
Cisco's Migration Tool allows you to convert specific features in an ASA configuration to the equivalent features in an Firepower Threat Defense configuration using the easy to use wizard based Migration Tool. Download the Migration Tool here . Release notes here.