When you have File and Printer Sharing enabled, for instance, a specific port will be open on your computer. This port exists to handle requests for access to your computer's files and printers. Hackers using a port scanner will see this port. Other applications on your computer may have ports open as well. Some applications may be invisible to you and may actually be Trojans.
Trojans are small programs that can provide a host of services to a remote user.
Trojans are typically installed by accident when a user downloads a file from the Internet, or opens one in their email. Like a virus, a Trojan can be transferred from one computer to another and is typically designed to hurt the computer on which it is installed. There are a number of types of Trojans, with new ones appearing each month. The simplest may just record keystrokes from your keyboard and stream them to a hacker. The hacker can then learn all of your passwords and any information you provide in email or via the web including personal information and credit card information.