Rats are neutral mobs that become aggressive at low light levels. Similar to Vanilla Wolves and Zombie Pigmen, they are Pack Mobs, so will attack the provoker of any member of their pack. They are not very dangerous, however, and have low health and damage values. They drop Coal upon death. Players might make the error of mistaking them for Mice, (which run away from the player instead of attacking) which are much less dangerous and easy to kill.
A Hell Rat is a hostile mob similar to the Rat. They only spawn in the nether and are red instead of brown. They are uneffected by fire and have more health and strength then the normal rat. They drop Redstone and Fire upon death.
Source Technic Pack Wiki