If the seelie court are the "good" fairies then these are the evil ones. The unseelie court is composed of the ugly, cruel fae that haunt the bad places of the world, that regard humans as enemies - or at least as subjects for their sadistic practical jokes.
If they look human at all, they will be disfigured and hideous and the more common ones that look like animals or insects will be far worse. A beautiful unseelie fae is likely to be something truly vile covered by an illusion of wholesomeness … or may just wear its corruption on the inside. Unlike their seelie rivals they are often solitary and antisocial, gathering infrequently for revels that are best not contemplated too closely.
Unseelie fae are best avoided wherever possible - their lairs will tend to have an unsavoury reputation and be places that people, children in particular, tend to go missing for no apparent reason. If they deal with humans, no good can be expected to come of it - any favours they do will tend to be expensive and poor value for the price and gifts will generally turn out to be sources of misery.
Tradition, which may be as old as Shakespeare, but may be much younger, names the Queen of the Unseelie Court as Madb (or Mab), sometimes called the "Queen of Air and Darkness". Her consort is commonly known as the Erlking and may or may not be called Madoc.
"The Other Wiki" on Seelie and Unseelie fae.