In User-centered interaction design, the concept of usability testing is described as a technique by which a product is evaluated by testing it by it's real users. This process gives gives a clear idea of how real users use the product. In contrast to that is usability inspection where a user interface or ui is tested using various usability inspection methods without any involvement of real users.
The intent of Usability testing is to measure the capacity of the product in meeting the intended purpose. In Human-computer studies, attempts are made to create universal principles where as this process measures the ease of use of the give product. Usability testing is a way to see how easy to use something is by testing it with real users.
There are a few different types of usability testing or reasons to conduct usability research:
Explorative Usability Testing
Before a new product is released, explorative usability testing can establish what content and functionality a new product should include to meet the needs of its users. Users test a range of different services where they are given realistic scenarios to complete which helps to highlight any gaps in the market that can be taken advantage of and illustrate where to focus design effort.
Comparative Usability Testing
Used to compare the usability of one website with another. Comparative tests are commonly used to compare a website against peer or competitor sites, however it can also be used to compare two designs to establish which provides the best user experience.