NetFlow is a network protocol developed by Cisco Systems to run on a range of network devices for collecting IP traffic information.

Previously only Cisco IOS routers and the Cisco 5580 ASA supported Netflow. But now with the introduction of Cisco ASA software 8.2, the complete ASA family now supports Netflow.
There are 3 event types that can trigger the creation of a Netflow record. These are `flow-create`, `flow-denied`, `flow-teardown`. You can also use `all` to trigger on any of these 3.

Below shows you an example of how to configure Netflow. In this example the Netflow server is location on the inside interface and has an IP of 10.1.1.10. Along with this all Netflow traffic will be sent via port 9998.
In this example I have added the class-map to the policy-map "global_policy". Which in most cases should already be applied to the ASA globally (all interfaces) via the service-policy command.

ASA(config)# flow-export destination inside 10.1.1.10 9998
ASA(config)# access-list flow_export_acl permit ip any any

ASA(config)# class-map flow_export_class
ASA(config-cmap)# match access-list flow_export_acl

ASA(config)# policy-map global_policy
ASA(config-pmap)# class flow_export_class
ASA(config-pmap-c)# flow-export event-type all destination 10.1.1.10

Additional resources : http://www.cisco-tips.com/cisco-asa-netflow-support-%E2%80%93-netflow-security-event-logging-nsel/