Virus

Virus:
A computer virus is a computer program that can copy itself and infect a computer. The term "virus" is also commonly but erroneously used to refer to other types of malware.
Viruses can increase their chances of spreading to other computers by infecting files on a network file system or a file system that is accessed by another computer.

Malware:
Short for malicious software, is software designed to infiltrate a computer system without the owner's informed consent.
Malware includes computer viruses, worms, trojan horses, spyware, crimeware, most rootkits.

Worm:
is a self-replicating malware computer program. It uses a computer network to send copies of itself to other nodes (computers on the network) and it may do so without any user intervention. This is due to security shortcomings on the target computer. Unlike a virus, it does not need to attach itself to an existing program. Worms almost always cause at least some harm to the network, if only by consuming bandwidth, whereas viruses almost always corrupt or modify files on a targeted computer.

Trojan Horse:
A Trojan horse, or Trojan, is malware that appears to perform a desirable function for the user prior to run or install but instead facilitates unauthorized access of the user's computer system. "It is a harmful piece of software that looks legitimate. Users are typically tricked into loading and executing it on their systems", as Cisco describes.
A Trojan horse may modify the user's computer to display advertisements in undesirable places, such as the desktop or in uncontrollable pop-ups, or it may be less notorious, such as installing a toolbar on to the user's Web browser without prior mentioning.
Once a Trojan horse has been installed on a target computer system, a hacker may have access to the computer remotely and perform various operations, limited by user privileges on the target computer system and the design of the Trojan horse

Spyware:
Spyware is a type of malware that is installed on computers and collects little bits of information at a time about users without their knowledge. The presence of spyware is typically hidden from the user, and can be difficult to detect. Typically, spyware is secretly installed on the user's personal computer. Sometimes, however, spywares such as keyloggers are installed by the owner of a shared, corporate, or public computer on purpose in order to secretly monitor other users.

Crimeware:
Crimeware is designed to perpetrate identity theft in order to access a computer user's online accounts at financial services companies and online retailers for the purpose of taking funds from those accounts or completing unauthorized transactions that enrich the thief controlling the crimeware.

Rootkit:
Rootkits can target the BIOS, hypervisor, boot loader, kernel or less commonly, libraries or applications.
The most common forms of rootkit either damage the systems they occupy or redirect the systems' resources for purposes ranging from pranks to gratifying their authors' egos to crime.

Virus vs. Worm:
A virus stays on your computer and your computer only.
Worms crawl through networks.
Unlike a worm, a virus cannot infect other computers without assistance. It is spread via trading programs with others (file sharing programs, email).

Spyware  vs. Adware:
Symptoms: Sluggish Pc, Increased pop-ups, homepage changes, strange search results.
Both are data miners, meaning they are looking for information. Both cause the above symptoms.
Can lead to identity theft.
9 out of 10 pc’s are infected.
Good place to look for info? http://www.webroot.com/spywareinformation

Spyware Removal:
You can try to do it manually- but is often very difficult. 
Often it can disrupt major computer processes.
You can check out the following free programs:
AdAware
SpySweeper (both versions, free&pay)
NOTE: Please be careful when downloading these tools, some programs claim to remove spyware, but instead, come with their own spyware embedded.

Top Spyware Threats:
PurityScan
N-Case
Gator
CoolWebSearch
Tansponder
ISTbar/AUpdate
KeenValue
Internet Optimizer (bargain buddy)
Perfect Keylogger
TIBS Dialer

For more information on these, visit:
http://www.webroot.com/spywareinformation/spywaretopthreats/