It may sound self-explicit, but here is a brief description of Quality Assurance:
Quality assurance (QA) is a way of preventing mistakes and defects in manufactured products and avoiding problems when delivering products or services to customers; which ISO 9000 defines as "part of quality management focused on providing confidence that quality requirements will be fulfilled". (source: Wikipedia)
What is the difference between Quality Assurance and Quality Control?
The main point of Quality Assurance (QA) is to prevent any defects before they occur. Therefore, Quality Assurance (QA) is a proactive activity by its nature. On the contrary, Quality Control (QC) aims to identify any possible issues and verify the quality of the output. Inherently, Quality Control (QC) is a reactive activity and it is conducted only after the Quality Assurance (QA).