Any computer or device connected to the Internet that is not properly protected is vulnerable to a variety of malicious Internet intrusions and attacks.
A firewall can be a hardware device, a software application or a combination of the two. Firewalls are designed to create a secure barrier between your computer and the rest of the Internet.
Firewalls can prevent malicious attacks and block certain types of data from entering your computer or private network. They can also be set up to alert you if anyone tries to access your system.
Cable modem and Digital Subscriber Line (DSL) users are particularly vulnerable to Internet intrusions and attacks because both connection methods provide "always-on" connection capability. The likelihood of a malicious individual accessing your computer increases significantly the longer your computer is on and connected to the Internet.
Your Internet Service Provider (ISP) may offer a firewall, or you may purchase firewall hardware or software from a reputable computer store.