It should not be identified with any particular political cause. It usually occurs in civil conflicts, and we associate its use with the weaker side, thus with groups who want to challenge powerful governments. But governments may also use terrorism against their opponents or against their own citizens. Terrorism typically involves deliberate violence against people who are not prepared to defend themselves and who pose no direct or immediate threat to those who attack them. Terrorist attacks are prepared in secret, the better to catch victims and the governments who would protect them off guard.
Surprise is essential to creating the psychological shock that makes terrorism effective.
Targets are selected for their symbolic value, not because killing or injuring them will necessarily make any material difference to the outcome of the struggle that has led to violent confrontation. It is the emotional reaction of the public to terrorism that counts, not its military value. To gain some perspective on the question of moral judgment, let me introduce a comparison. In warfare, it can be that the justice of the cause gives an actor greater moral standing, so that possibly we could excuse means that we would otherwise condemn. But we would require that the need to use morally abhorrent methods be desperate and that the user of such methods have no other alternative.