Action Message Format (AMF) is a serializing format like XML and JSON. Where the latter two sends formatted text, AMF sends binary data. This allows one to transfer your own domain objects instead of creating a data transfer scheme for XML or JSON. BlazeDS, an open source project from Adobe, is the 'enabler' for AMF.
This site compares the data loading performance between AMF, XML, JSON and a few others. The screenshot shows that AMF took the least time and bandwidth to load the data.
Sites of Interest:
BlazeDS and what it means for the developer community
BlazeBench: Why you want AMF and BlazeDS
Ajax and Flex Data Loading Benchmarks