What's the difference between HSDD and asexuality?
Hypoactive sexual desire disorder, or HSDD, is a disorder where a person doesn't experience sexual fantasies or desire for sexual activity. Superficially this sounds very similar to asexuality, but to classed as a disorder, it must cause marked distress in and of itself. Although asexuals may experience negative effects as a result of their asexuality, they do not feel a need to 'cure' their condition arising from inside themselves.
Additionally, those with disorders like HSDD typically experienced desire at some point, and then lost it. Asexuality on the other hand is usually lifelong – most asexuals report never having experienced sexual attraction in their lives.
Asexuality is officially not a disorder according to the DSM-5, which is the latest edition of the American Psychiatric Association’s Diagnostic and Statistical Manual of Mental Disorders. You can find screenshots and a discussion of this topic here.