Agent.APB is a backdoor that installs a proxy server, and opens a random port on the affected computer.
Then, it makes GET requests of a specific format to three different websites in order to notify its authors that the computer has been compromised and supply them with the necessary information to access it remotely: number of the port opened, version of the operating system, last known IP address, etc.
Additionally, one of the components of Agent.APB provides it with the ability to make downloads, so that it can update itself.
Agent.APB does not spread automatically using its own means. It needs an attacking user's intervention in order to reach the affected computer. The means of transmission used include, among others, floppy disks, CD-ROMs, email messages with attached files, Internet downloads, FTP, IRC channels, peer-to-peer (P2P) file sharing networks, etc.