A computer virus is a program or piece of code that is loaded onto your computer without your knowledge and runs against your wishes. Viruses can also replicate themselves.
Phishing is a form of fraud in which the attacker tries to learn information such as login credentials or account information by masquerading as a reputable entity or person in email, IM or other communication channels.
Pharming is a scamming practice in which malicious code is installed on a personal computer or server, misdirecting users to fraudulent Web sites without their knowledge or consent. Pharming has been called “phishing without a lure.”
Malware is an abbreviated term meaning “malicious software.” This is software that is specifically designed to gain access or damage a computer without the knowledge of the owner
Well for the most part there are thousands of low life criminals out there always active, always looking for victims. These people are relentless and are very skilled and convincing at what they do so knowing how to prevent this is a vital thing to know because you never know, you could be there next victim.
There are a number of reasons why people would do this. In short this is a numbers game. If a very small fraction of the people fall victim to them it could be a substantial return for the criminals.