What Does Remote Access Trojan (RAT) Mean?
A remote access Trojan (RAT) is a program used by the intruders to take complete control of the victim's computer for the purpose of performing various malicious activities. Unlike viruses and worms, RATs can exist well before detection and even remain after removal. They operate in a stealth mode and are usually rather small so as to avoid detection.
Techopedia Explains Remote Access Trojan (RAT)
A RAT's installation mechanism is usually attached to a legitimate program. RATs are commonly hidden within games or other small programs, as well as in email attachments that users download. Imposters are able to customize RAT features, such as when and where to launch the Trojan. Intruders eventually trigger the server program on the victim's computer. Then, the RAT runs invisibly on the victim's computer and allows the intruder to gain remote access and control of the infected computer.
Two of the most popular RATs are Sub Seven and Back Orifice. The Cult of the Dead Cow created Back Orifice in 1998 and released it under the GNU General Public License.