What Is a Bug?

A Bug is an error,fault or mistake that doesn't allow the program to behave in predefined and desired manner and it starts behaving in unintended manner.
There are different kinds of bugs, small bugs may cause of non performance of a small part of a program, for example: while pressing menu button one can not see the menus. There could be huge bugs that can even crash the the whole program & system.

Before releasing any software to the end user, it has to go through any levels of testing. 
So,the process of finding and fixing such bugs is known as "Debugging".

After rigorous testing , Testers give a detailed report about such bugs to the developers to fix it.

Causes of Bugs:

• Miscommunication & Misconception: In the very first stage of requirement gathering, at times there are miscommunications between the stakeholders & Developers & testers. If this went well, at times the visualization and conceptualization goes wrong that creates bug in the software.

• Erroneous Program Designs :

• Erroneous Source Codes: If developers starts coding in haste without having a clear understanding of the requirements. Or the developer does not have the expertise in that particular domain. 

• Changing Requirements: Clients keep on changing their requirements, developers change their codes but that can have buggy effect at some other places in software.

• Third Party Tools: Many tools are used to develop a software like compilers, Visual tools,Scripting Tools may have their own bugs that gets added to the given software.