Validation testing provides answers to questions such as:
Does the software fulfill its intended use?
Is the company building the right product?
Can the project be properly implemented?
Are the documents in line with the development process?
Validation Testing Process
The aim of validation testing is to measure the quality of a software in terms of number of defects found in it, the number of tests run, and the system covered by the tests. When bugs or defects are found with the help of testing, the bugs are logged and the development team fixes them. Once the bugs are fixed, testing is carried out again to ensure that they are indeed fixed, and no new defects have been introduced in the software. With the entire cycle, the quality of the software increases.
Read more at Buzzle: http://www.buzzle.com/articles/validation-testing.html
UAT testing is done by the client.
For example client requirement is that the application which we have developed should work properly on Windows 2000 as well as windows XP .This is one of the UAT testcase.