Autonomous System Numbers are used in the routing system in the Internet.
A simple description of an AS number is a number every ISP needs to refere to its own routes in the global routing table.
As not only ISPs needs AS numbers but every network with a separate routing policy, the original 2 byte AS nubers were not enough and the routing protocols has been updated to allow for 4 byte numbers.
The new global policy describes how ICANN/IANA will allocate blocks of 2 byte and 4 byte AS Numbers to the RIRs.
The ICANN board ratified this policy on July 31 2008: http://www.icann.org/en/topics/policy/update-aug08.htm#15