ALF and ELF cannot win through moral suasion nor are they able to form a united opposition to animal testing and logging. They must therefore resort to tactics aiming to intimidate others through threats of violence and destruction. Since their aim and tactics are to terrorize others to prevent them from testing on animals or logging, they are terrorist organizations.
Yes, the ELF and the ALF are terrorist organizations, because that is how the United States has classified them. These groups, called liberation fronts, openly advocate for warfare and economic meddling in order to push their agenda, which is to impose environmentalism by force. They even want to destroy buildings. This is terrorism.
The definition of terrorism is a slippery one, and has been used in many different ways by many different organizations over the years. The ELF and ALF groups are no exception to this rule. It is difficult to say whether or not the attribution of terrorism to their activities is accurate.
ELF stands for Earth Liberation Front and ALF stands for Animal Liberation Front. ELF has been quoted as supporting, "economic sabotage and guerrilla warfare to stop the exploitation and destruction of the environment". ALF is mainly involved with animal rescues, in a somewhat vigilante sense. The organizations are separate and developed apart from each other.