@amy

a few mistakes:
Worms –
Worms can replicate themselves from one machine to another """without the need of downloading them from the internet"""". """They often send themselves as attachments in emails ""they generate from their infected host computer.

Here you should be clear about the meaning of the terms "downloading" and the system of emailing..the quoted words are going against eachother, when u r using or opening an email, it means that u r downloading information from the Server thats providing you email services....

when a program is moving from one system to another it then means that it is using the internet and thus is uploaded and downloaded.

A computer worm is a self-replicating computer program. It uses a network (meaning it can upload and download itself through internet from one computer to any other,thus using internet downloading pocess, or from any server to any client or vice versa) to send copies of itself to other nodes (computer terminals on the network) and it may do so without any user intervention. Unlike a virus, it does not need to attach itself to an existing program, because it can propagate without the need of any host. Worms almost always cause harm to the network, if only by consuming bandwidth, whereas viruses almost always corrupt or modify files on a targeted computer.

TROJAN HORSE:
trojan horse is actually a malicious computer code which pretends to perform a useful task but infact its functions are destructive. it can act as a virus or worm depending heavily on the form of code in it..
the heading uve mentioned separately come under trojan horse:
1. backdoor.
2. DoS (denial of service, means trojan causes the network to get so heavily loaded that the end users can't get the desired data from the servers, and hence named denial of service..the bandwidth available is consumed by the trojan to do destructions)

3.not only downloads but also uploads.
4. dialers are infact the "URL Trojans"
5. remote access (allowing a faraway system to get acess into your system, by sending a trapdoor which finds eak holes in the kernel of the operating system to make an attack easier)
6.email sending and making its contents diffferent according to the will of the attacker, such as the signature of the real owner of the email id.
7. logging (making a list of or storing the record of the keystrokes made by the user on the keyboard) keystrokes to steal information such as passwords and credit card numbers.
8. trojans also disable the security softwares or the firewall installed on ur system.
9. opening and closing of your CD-ROM tray automatically.

Since trojans have many different forms, they cannot be deleted by using only one type of anti-malicious software, normally a good antiirus can be used to delete them, by deleting the temporary internet files, or by running your system in a safe mode and deleting the trojans manually which normally reside in the registry of the computer system. you can be safe somehow from the trojan destructions by not opening the unknown emails, and other insecure websites because it infects the system through emails, instant messaging (chating), through floppy disks and flash memoies etc...

SO BE CAREFUL!