The primary difference between a comet and an asteroid is their assumed points of origin and their general makeup. Asteroids....for the most part, have little or no free water in the form of ice. Some maybe chocka block full of water, but it's contained within their minerals. Comets have a large proportion of their structure made up of ices in various states of crud....from almost pure to thick mud
Quite a few asteroids have orbits very similar to comets....some probably are old comets that have completely de-iced and what you have left is the rocky bits hanging about in the old orbits.