Viruses - Worms - Trojans >>>> malware

Many people use the term "virus" loosely to cover any sort of program that tries to hide its possibly malicious function and\or tries to spread onto as many computers as possible.

But

• What exactly is a Virus ?
• What are the others - Trojan Horses or worms ?

What make them different from others ?

Lets try to get a brief introduction about all these concepts here in this review ....

1. Computer Virus

A computer virus is a self-replicating program containing code that explicitly copies itself and that can "infect" other programs by modifying them or their environment such that a call to an infected program implies a call to a possibly evolved copy of the virus.

Computer viruses are actually a special case of something known as "malicious logic" or "malware".
It is important to understand the distinctions between viruses and the other forms of malware.

There are two main classes of viruses
1)File infectors
2)SYSTEM or BOOT-RECORD INFECTORS

FILE INFECTORS :
They attach themselves to ordinary program files.
These usually infect arbitrary COM and/or EXE programs

SYSTEM or BOOT-RECORD INFECTORS:
these viruses infect executable code found in certain system areas on a disk.

Aside from the two main classes described above, many antivirus
researchers distinguish either or both of the following as distinct
classes of virus:

FILE SYSTEM or CLUSTER viruses - those that modify
directory table entries so that the virus is loaded and executed before
the desired program is.

KERNEL viruses target specific features of the programs that contain the "core" (or "kernel") of an operating system

A TROJAN HORSE is a program that does something undocumented that the programmer intended, but that some users would not approve of if they knew about it.

2. Worms

Computer WORM is a self-contained program (or set of programs), that is able to spread functional copies of itself or its segments to other computer systems (usually via network connections).

Worms do not need to attach themselves to a host program. We will study about worms in more detailed way.

A computer WORM is a self-contained program (or set of programs), that is able to spread functional copies of itself or its segments to other computer systems (usually via network connections).

There are two types of worms--host computer worms and network worms.

Host computer worms are entirely contained in the computer they run on and use network connections only to copy themselves to other computers. Host computer worms where the original terminates itself after launching a copy on another host (so there is only one copy of the worm running somewhere on the network at any given moment), are sometimes called
"rabbits."

Network worms consist of multiple parts (called "segments"), each
running on different machines (and possibly performing different
actions) and using the network for several communication purposes.
Propagating a segment from one machine to another is only one of those
purposes. Network worms that have one main segment which coordinates
the work of the other segments are sometimes called "octopuses."

unlike viruses, worms do not need to attach themselves to a host program.