Sub7 (also known as Backdoor-G and all of its variants) is the most well known Trojan / backdoor application available. As far as hacker tools go, this one is one of the best.
Sub7 arrives as a Trojan. According to Internet security firm Hackguard, these are the statistics for how one might come to be infected with a Trojan horse program:
Download an infected email attachment: 20%
Download an infected file from the Internet: 50%
Get an infected file on a floppy disk, CD or network: 10%
Download because of an exploited bug in Internet Explorer or Netscape: 10%
Because of its many uses, you may receive it from someone you would normally trust- a friend, spouse or co-worker. By virtue of being a Trojan horse program it comes hidden within a seemingly legitimate piece of software. Executing the software will do whatever the application is supposed to do while installing Sub7 in the background.
Upon installing Sub7 will open a backdoor (enabling a port that you are not aware is open) and contact the attacker to notify them that Sub7 is installed and ready to go. This is when the fun begins (for the hacker at least).