Named after the ancient Greek wooden Trojan horse, Trojans are malicious programs hidden within, or disguised as, legitimate software or files. They may secretly steal your data and harm your system, while masquerading as legitimate software. For example, you might be playing a game but the software is stealing your data in the background.
Remote access Trojans are often used to watch you through your webcam, record typing from your keyboard, including credit card numbers and passwords, or open a backdoor for unauthorized access to your computer. You should cover your webcam camera with a piece of paper when it is not in use.
Trojans will often present themselves as useful or required normal software, or as updates for programs. Only download and install software from trusted sources. Avoid "cracked" or free versions of programs from secondary sources. Set your antivirus software to automatically scan all programs and files downloaded to your computer.