What is a dead comet? How is a Dead Comet different from the normal comet? 

How are they formed?

And why is the Halloween asteroid 2015 TB145 called a dead comet?.
A comet is usually characterized by its tail. A dead comet has lost all its ices and gases (responsible for producing this tail), leaving just a rocky core. 

The Halloween comet is such a dead comet, in that it has no tail, but furthermore it resembles a skull, making it particularly relevant for Halloween.