Virus is a self-copier program which attaches itself with an executable program and sometimes hidden behind with executable applications and When the file is command for execution the virus automatically executed and attack in the system memory. Just once it go into system memory it will start its work as I have mentioned before that it is self replicating program means searches other exe extension files which can be infected or it resides in the background and infect the files.
Worms are extremely comparable to that of the Virus however it is different in the way that they don’t tie themselves to executable programs rather to reproduce themselves utilizing the networks. The aforementioned worms dependably focus on your network. So, if you find exorbitant usage of your network capacity then you may be tainted by a worm. A worm don’t need a command to execute any program for its execution it can work without users involvement. So this is much most dangerous than the virus.
A Trojan is also unsafe and Harmful program which might appear innocuous to the user before its usage. However, rather, it is customized or invert designed to help unauthorizedly remote access to the other computers. Trojan’s don’t imitate them self rather they give access to your documents to the remote computer/user who designed that Trojan horse.
Spyware may be most dangerous than all of above it is the program that keep an eye and gather information from pc secretly. It usually run in secrecy mode and very difficult to detected easily. As you have already heard about Keyloggers they are enormous example. There are no longer limited to only send data to the remote computer but also spying.
Learn How to Protect Your Computer from Virus Trojan Adware and Spyware. See This Post