Software Quality 2: What?

Okay, so we know *why* we want our software to be of high quality. The bottom line is that bad quality will, in the end, result in no one buying/using it. That's bad for a generic enterprise where employees don't use the enterprise software but even worse for a software company; if people don't buy your stuff, Captain Obvious states: you'll go bankrupt.
But knowing why we want to do it doesn't make it happen. To be able to improve software quality we first need to know what it is. What is software quality? How do we know our software is of high quality?