ARM - which stands for Animal Rights Militia - is a faceless organisation comprising of many radical animal rights groups working together in support of a shared ideology: what makes the ARM different from direct action groups such as the Animal Liberation Front (ALF) is that members of ARM don't take precautions to ensure people don't get harmed - in fact many members of ARM belief in utilising terrorist tactics and for this reason the movement is seen as much more dangerous.
Tactics utilised by those who follow the ARM include (but are not limited to) arson, letter bombs, contanimation, death threats and vandalism - although these groups tend to target hunters, animal testing labs and other obvious targets they have also attacked other targets: for example one of the groups associated with ARM sent a letter-bomb to No 10 Downing Street (the abode of the British Prime Minister) - this could easily qualify as attempted assassination and simply serves to further the group's infamy.
The philosophy of ARM is a simple but extreme one - simply put members of ARM do not care if they have to endanger human life in the pursuit of animal welfare and are prepared to utilise far more aggressive tactics to further their goals.