What do Viruses, Trojans, and Other Malware Actually Do?

Viruses: The term comes from the biological term, virus. Like virii in real life, a computer virus infects an organism and then spreads by replicating itself. 

Trojans: The term is again borrowed from the concept of the "Trojan Horse" from Greek Mythology. A Trojan is a computer application which looks completely harmless on the outside. It is only after you install it that you realize what it actually is, but the damage is done by then.

The amount and extent of damage caused by any malware depends greatly on its purpose. Some malware were written only for fun and may display a limerick on your screen, but nothing more. Others may harvest your data and send it out so that it can be sold to the highest bidder. Some malware exist only to break into some other program in your system and then download its payload which is what actually performs the malicious activities. 

Some malware will do absolutely nothing to your system. Stuxnet is a famous example of this. It was written to target only a certain computer and would remain dormant on all others.

Yes, you can always remove a malware from your computer manually. You only need to replicate the exact steps that the anti-malware applications perform. It's possible, I've done it, but it's not easy. And more often than not you'll leave something behind. You should use the specialized softwares available instead.

The easiest way to prevent malware from infecting your system is to use Linux. As explained by Miami Tom Windows has an inherently broken security mechanism. Linux on the other hand is very strong. If you must stay on Windows, use a good anti-malware tools with a firewall.

- Trojan is a server program that comes binded inside non-official software installers to be installed as well as the wanted program: and then it gets full administrator rights to load on startup, hide itself from the list of processes, bypassing antivirus and firewall .. and finally do a reverse connection to the cybercriminal machine that can do all with the remote infected machine : and thats from what its name comes.

- Virus is often a malware that replicates itself to many machines exploiting the autorun vulnerability, but also can be hidden inside other software to make disruptive changes inside the computer without asking the user, after being executed.

- Other malwares can be worms (virus attached to email message) and adwares (software that displays ads).

is a type of malicious software, often confused with viruses or other parasites. The Trojan horse is a seemingly legitimate software but containing a maliciously. The role of the Trojan horse is to get this parasite on the computer and install without the knowledge of the user

A virus is a small infectious agent that replicates only inside the living cells of other organisms. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea.

Viruses - replicate themselves. 

Trojans - trick users into installing something bad. 

Malware - is a catch-all term for anything bad on your system, but is usually used for things that don't fit into other categories. 

All of them drop something unwanted on the target computer.  Viruses and Trojans usually have some sort of "use your computer without your permission" or "harvest your information" package.  Malware just does things with your computer that you didn't ask it to - like changing your browser's home page or search engine, or popping ads up, or something else that uses your computer for things you didn't ask it to do.