User Acceptance Testing is -

official tests carried out on the system to ensure it meets the acceptance criteria, before the system is put into production. [Most of the time it is made by the users / clients]
the incremental process of approving or rejecting the system during development and maintenance.

Acceptance Testing checks the system against user requirements. It is made by real people using real data and real documents to ensure ease of use and functionality of systems. Users who understand the business functions run the tests as specified in the acceptance test plans, including installation and hard copies on-line help of user documentation are also being reviewed for ergonomics and accuracy. Testers / users to formally document the results of each test, and provide error reports, requests for correction for developers.

Myth in user acceptance testing - Passing the AUT recognizes that the system is fit to use and it also recognizes the development process was good enough

Now a days we use Agile and incremental software development models. While acceptance testing should be the current activity. It needs to be involved in the development process and need to be approximate correction made ​​whenever it fails the acceptance criteria.

During Acceptance Testing software allows:

• Early detection of software problem.
• Early the needs of users when developing software.
• Ensure users are involved in the scheme criteria and acceptance.
• Decision involved based on the results.