Ghosts are the spirits of the dead that can appear or manifest in visible form to the living.
Aside from helping Shadowhunters and mundanes to see past glamour, The Sight also grants the ability to see, hear, and speak with ghosts that choose to be seen or heard, but only some Shadowhunters, like Will Herondale, are capable of interacting with all of them, even those that do not want to be seen or even heard. Others can only sense their presence through the unnatural cold feeling they exude. Ghosts, like vampires rising from their graves, draw energy from their surroundings to maintain their ectoplasmic form when they manifest in the world, thus sucking the heat from their surroundings.
The eyes of all ghosts are hollow and empty. However, according to Shadowhunter belief, in very rare cases, some strong spirits have flames flickering in their eyes. For some reason, it also takes a while for ghosts to move beyond the place where they reanimated, usually the very spot where they died.
Many, though possibly not all, ghosts are tied to the living world by a personal effect or an unresolved wrong. Trapped for years, the ghosts cannot fully pass on until they find the item, often a lost sentimental piece of their past, or perform the act that might put them to rest. Some of these restless ghosts are condemned to forever seek out their salvation, not knowing what they need, or unable to accept that their chance has passed, as in Old Molly's case, whose ring was believed to be long gone.
Meanwhile, other spirits simply choose their fate and decide themselves to stay in the world instead of passing over.