What is malware software? How it causes to a computer? How to get rid of that?

Do you mean "Malware software"?

Malware is the term given to programs or viruses that cause the PC to go nuts!

Other times you will see the term with such products as "Malware bytes". This is software that will remove viruses and other pieces of malware from your PC.

Don

Malware, short for malicious software, is software designed to infiltrate a computer system without the owner's informed consent. The expression is a general term used by computer professionals to mean a variety of forms of hostile, intrusive, or annoying software or program code.[1] The term "computer virus" is sometimes used as a catch-all phrase to include all types of malware, including true viruses.
Software is considered malware based on the perceived intent of the creator rather than any particular features. Malware includes computer viruses, worms, trojan horses, most rootkits, spyware, dishonest adware, crimeware and other malicious and unwanted software. In law, malware is sometimes known as a computer contaminant, for instance in the legal codes of several U. S. states, including California and West Virginia.[2][3]
Malware is not the same as defective software, that is, software that has a legitimate purpose but contains harmful bugs.
You can get more details at: Malware - Wikipedia, the free encyclopedia
Joyhong software

Others have done agood job of explaining what malware is, ow how do you git rid of it?
I will assume you are running Windows because you are asking about malware, first install a good antivirus and keep it up to date Nod32 is a favorite of a friend who evaluates them for a living. Keep your system up to date. Microsoft offers a program Windows Live OneCare that I have heard good things about.
The other part of stopping malware is to increase your security, some of this is technical the rest behavioral things like use a firewall and be behind a router will help. Also being more aware of the risk you are exposing yourself to and eliminating or lessening them.
Because of it's market share Windows and Internet Explorer are popular targets, so your risk of infection is increased by using them. Operating systems such as Mac OS 10 and Linux offer less or a target and a lower instance of exposure. In addition these Unix based or inspired systems also do not run Windows code or programs and offer an added protection of allowing administrator functions only when needed and not as the default mode of operation. In the case of Mac and Linux you will usually need to enter your password to enable you to make changes effecting your operating system.
For removing bad malware infections from Windows computers I prefer the scorched earth approach. This involves backing up your data, reformatting and partitioning your drive installing a anti virus program,running your system updates until there are no more updated to install then scanning your backup for infection repairing any infections and moving your data back to the computer. Then defragmenting your hard drive and running a good registry cleaner.
If you keep your OS updated as well as your anti virus software. With a clean install and protection in place you should stay clear of infection assuming you operate the computer while mindfull of the risk.
Many risk can infect your computer through more elaborate attacks that take advantage of errors in some installed program to allow code to be loaded and executed your best defense against this attack is to be updated .
Good luck and happy computing