Malware in short known for malicious software. It is a software designed to infiltrate a computer system without the owner's informed consent.

Malware includes computer viruses, worms, trojan horses,  rootkits, spyware, dishonest adware, crimeware and other malicious and unwanted software.Around 80% of malware today is designed to find and steal confidential information stored on your computer. This type of malware is sometimes called “crimeware.”Malware can invade your machine through infected email attachments, “bots” that crawl the Internet looking for unprotected computers, and visits to “hostile” Web sites.
Virus

A computer virus is a program which is able to replicate and attach itself to a program or files infecting the system without our knowledge. The software programs that hide on your computer and cause mischief or damage.

Sign of Virus

• Computer runs more slowly than normal
• Computer stops responding or locks up often
• Computer crashes and restarts every few minutes
• Computer restarts on its own and then fails to run normally
• Applications on your computer don't work correctly
• Disks or disk drives are inaccessible
• Can't print correctly
•  See unusual error messages
•  See distorted menus and dialog boxes

Spyware

Spyware is a generic term for malicious software which ends up on your computer, and is used to gather information about you and other files on your computer and passes it over internet to others.Generally speaking, spyware is software that hides on your computer, tracks what you’re doing online, and then sends that information over the Internet. Some types of spyware, called “keystroke loggers” actually record and send everything you type on your computer.Spyware software can sneak onto your computer when you download unsafe software and files—or even visit a hostile Web page.One major source of spyware is the peer-to-peer file sharing software commonly used to share music and videos online.

Worm

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 and it may do so without any user intervention. This is due to security shortcomings on the target computer or  by exploiting vulnerabilities in operating systems. Worms almost always cause at least some harm to the network.

Trojan

It is a non-self-replicating malware that appears to perform a desirable function for the user but instead facilitates unauthorized access to the user's computer systemTrojans are backdoors in to your computer in which access is gained by hackers and gain remote access to a target computer system to perform various operations.The operations that a hacker can perform are limited by user privileges on the target computer system and the design of the Trojan horse  on the internet to either gather information from your computer or to use your  computer.