What exactly is a Desire?

What exactly is the anatomy of a Desire and why does it have such a potent effect on our psyche and life? Is it a ***thought*** of wanting something we don't have, but then we have so many thoughts about so many things? Is it a ***feeling*** of missing something or something being wrong in the present moment, but then we have so many other feelings? Is it a mental reaction, or ***perception*** of thought and feeling? Is it some mental energy illumined by ***consciousness***? Or is it will want? The Buddha went as far as saying, ***taṇhā*** ("craving, desire or attachment") is the cause of suffering. I want to understand what is this as a phenomenon?