A Sheer is a soul who haven't been able to cross over to the Otherworld because they have unfinished business in the Caster or Mortal world, such as a powerful curse, lost love, a shattered destiny, and etc..
Ethan Wate, Wesley Lincoln, and Olivia Durand saw a lot of Sheers at Exile, where Liv gave a more clarification of what they were to Link. There were many Sheers at Exile because they were responding to the Vex..
They can pick stuff up, move furniture, slam doors, float off the ground, and kelt, as seen with Genevieve Duchannes. They have specific, seperate casts like the Umbra Cast which sends a shadow from one world to the next and the Veritas Cast which allows the truth to appear as stated by Xavier.