Why do we test? What does QA do?

Recently I was asked for a definition of QA testing and what happens in QA in 1 to 3 sentences:

QA tests and checks software functionality, performance, integrations, and configurations.  We test to demonstrate that the software and system are "Fit-for-Use".  Testing and checking activities include requirements review, test planning, test execution, test progress and status reporting, development of test assets (diagrams, test cases, automation), and defect management.

Failures, errors, faults, defects, and bugs...

The Junit test framework TestResult class distinguishes between failures and errors.  The distinction is a failure is anticipated and an error is unanticipated.  I think this is a helpful distinction that can be used in general when describing problems we see when testing software.  Now, just need to do the same with faults, defects, and bugs.

"The test framework distinguishes between failures and errors. A failure is anticipated and checked for with assertions. Errors are unanticipated problems like an ArrayIndexOutOfBoundsException."