Virus types
Viruses are traditionally classified into two major categories: boot viruses, which infect the boot area of floppies and hard disks, and become resident and active at the time of booting the machine. The other catgory is file viruses, which infect one or more types of program files, and activate when the program is run.
Until the mid-1990s the most common infections were of boot viruses, even though a huge majority of the known viruses were file viruses. This was because, in order to spread far, a virus needs a means of get from one machine to another. Floppy disks are commonly shared (although nowhere near as commonly now as in the earlier days of the "PC revolution"), and while not all disks carry program files (reducing the chances for file infectors to spread), a diskette always has a boot sector and thus, potentially, a boot virus. Dependence on the common human practice of exchanging diskettes (and occasionally forgetting to remove them from the default A: boot drive) was an efficient means for a virus to spread when "sneakernet" was the common means of file sharing and software distribution.

What is an E-mail Virus
Most viruses are in fact email virus. The term "email virus" reflects the way the virus spreads, so there is no specific virus type called e-mail virus.
Most virus spreads via email, i.e. the Loveletter, Nimda, Klez and many others. Most of these use Microsoft Outlook to send itself to all addresses in the address book, usually spoofing the senders address, which makes the reciever more indulged to open the mail and execute the infected files.

Computer worm
Worms are very similar to viruses in that they are computer programs that replicate functional copies of themselves (usually to other computer systems via network connections) and often, but not always, contain some functionality that will interfere with the normal use of a computer or a program. The difference is that unlike viruses, worms exist as separate entities; they do not attach themselves to other files or programs.
What is a computer worm
Because of their similarity to viruses, worms are often also referred to as viruses.
A well-known example of a worm is the ILOVEYOU worm, which invaded millions of computers through e-mail in 2000. As intranets and the Internet have grown in popularity, e-mail has evolved from a convenience to a necessity. Virus vandals know that, and they've invented new ways to use e-mail to spread viruses, and especially, worms. A worm program is similar to a virus. It is considered by some to be a subset of a virus in that it makes copies of itself but does so without needing to modify a host. Like viruses, worms may (or may not) do things other than replicate

What is a Trojan Horse Virus
Trojan horse programs are named for the giant wooden horse that concealed Greek soldiers who used it to invade the ancient city of Troy. Trojans are also called RAT (remote access trojan, or remote access trapdoor) Like that famous trick, a Trojan horse program conceals hidden programming. The hidden function may just be a joke, or something annoying, but vandals often use Trojan horse programs to destroy other people's data, knowing that some people will run any program that has an interesting file name, or promises to perform a useful function.

What is a Macro Virus
A macro is a piece of code that can be embedded in a data file. Some word processors (e.g., Microsoft Word) and spreadsheet programs (e.g., Microsoft Excel) allow you to attach macros to the documents they create. In this way, documents can control and customize the behavior of the programs that created them, or even extend the capabilities of the program. For example, a macro attached to a Microsoft Word document might be executed every time you save the document and cause its text to be run through an external spell checking program.
A macro virus is a virus that exists as a macro attached to a data file. In most respects, macro viruses are like all other viruses. The main difference is that they are attached to data files (i.e., documents) rather than executable programs. Many people do not think that viruses can reside on simple document files, but any application which supports document-bound macros that automatically execute is a potential haven for macro viruses. By the end of the last century, documents became more widely shared than diskettes, and document-based viruses were more prevalent than any other type of virus. It seems highly likely that this will be a continuing trend. One example of a macro virus is the Melissa virus.

Boot sector virus
Boot sector viruses place their code in the disk sector whose code the machine will automatically execute when booting. Thus, when an infected machine boots, the virus loads and runs. After boot viruses are finished loading, they usually load the original boot code, which they have previously moved to another location, or take other measures to ensure the machine appears to boot normally.

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