Multitasking
The ability of an operating system to run two or more tasks at once. With one processor you will not normally have more than one task using the processor at a given moment in time, but the tasks will be scheduled so that they can all appear to be running at the same time and do not interfere with one another.
Network
A group of interconnected computers. The computers must be capable of transferring data to form a true network
Protocol
A general behavior that computers and network devices must follow to understand one another. Think of it as a language. If two computers don’t use the same network protocol then they cannot communicate. Network protocols define such mundane things as signal timings, whether acknowledgement is needed, and packet sizes.
Trojan Horse (Trojan)
A computer program that appears to be something useful, but then does something malicious to your computer. This could range from destroying data to laying dormant and someday hijacking your computer to be used as part of a Denial of Service attack. Anti-virus programs will protect you from known Trojan horses, but strictly speaking Trojan horses are unlike viruses because they do not replicate. However, combination of virus/Trojans can replicate.