Requirement: Is a need given by customer/end user/stakeholders in company in written document 

Module: Component of a larger system that performs a specific function
Ex: Gmail application 

Defect: In Software Testing, Defect is an failure or error occurred in Software Applications

Bug: A bug can cause the software to behave in unexpected ways, such as producing incorrect results, or crashing other types of behaviour that deviate from the expected outcomes.

Error: An error can be caused by a defect or bug in the software, or by incorrect use or input from a user and can be fixed through the development and testing phase.

Cost of fixing defects: If defects are discovered after the software has been released, it can be much more expensive to fix them.

Functional and Non Functional Requirements: 

Functional requirements describe what a software system should do and what functions it should perform. They specify the behaviour of the system and the inputs and outputs that the system should produce. 
Examples of functional requirements include:

  • User authentication
  • Data storage and retrieval
  • Report generation
  • Online shopping cart

Non-functional requirements describe the quality attributes or constraints that a software system should possess, such as performance, security, usability, reliability, and scalability. Non-functional requirements do not describe specific behaviour, but rather set limits or boundaries on how the software should behave. 
Examples of non-functional requirements include:

  • Response time for a web page to load
  • Security standards for data protection
  • Maximum number of concurrent users
  • Availability of the system 99.9% of the time.

Regression testing: 

Stakeholders: Individuals brings up with business requirements