Computer viruses can be classified into several different types. The first and most common type is the virus which infects any application program. On IBM PC’s and clones running under PC-DOS or MS-DOS, most programs and data which do not belong to the operating system itself are stored as files. Each file has a file name eight characters long, and an extent which is three characters long. A typical file might be called “TRUE.TXT”, where “TRUE” is the name and “TXT” is the extent. The extent normally gives some information about the nature of a file—in this case “TRUE.TXT” might be a text file. Programs must always have an extent of “COM”, “EXE”, or “SYS”. Under DOS, only files with these extents can be executed by the central processing unit. If the user tries to execute any other type of file, DOS will generate an error and reject the attempt to execute the file.

When we think of Trojans, we normally think of the huge wooden horse that the Greeks gifted the city of Troy, in which they had secretly hidden a small number of their warriors. When the Trojans took the horse into the city the soldiers emerged and opened the gates to the cities enemies. Computer trojans are based on the same idea; what can appear to be useful program or harmless site can be hiding a small Trojan program that once it has infected your computer opens a secret backdoor that will allow an intruder to remotely gain access to your computer. Trojans are not viruses in that they are not designed to reproduce themselves. Instead they rely on being small and deceptive to propagate themselves across the internet, usually attached to some other file.

Every Window object has a document property that refers to a Document object. The Document object is arguably more important than the Window object itself: while the Window represents the browser window, the Document object represents the HTML document that is displayed in that window. The Document object has various properties that refer to other objects which allow access to and modification of document content. The way that document content is accessed and modified is called the document object model, or DOM, and there are several DOMs in existence:

Linux was developed as a freely distributable version of UNIX. UNIX is the most widely used operating system in the world and has long been the standard for high-performance workstations and larger servers. UNIX, first developed in 1969, has a strong programmer-oriented user group that supports the operating system.How did UNIX get its name? It was based on an operating systems called MULTICS (MULTiplexed Information and Computing System). Ken Thompson, Dennis Ritchie, and Brian Kernighan were involved the design of a new operating system based on MULTICS that would be much simpler. They called the new operating system UNICS (Uniplexed Information and Computing System), which was quickly changed to UNIX.