Aikido helps you meet a violent attacker and introduce them to the floor. You learn how to prevent harm - harm to potential victims, and harm to the attacker.

Aikido teaches some physical control, so you can be empowered to make decisions when situations reach a point of dangerous flux.

Aikido was invented in the early-mid 20th century by Morihei Ueshiba, called O'Sensei by his followers. Howard equates O'Sensei with Buddha and Jesus - O'Sensei studied a wide range of marital arts and came through them to see that the best conflict resolution was peace. So he calls Aikido a "budo of peace." You might think of Aikido as an evolved form of martial arts - evolved in favor non-violent, non-injurious resolution of conflict.

It verges on the spiritual - you learn how to anticipate people's energy, and plan the appropriate reception. These are principles you can practice facing a training partner, a mugger, a lover, a tax auditor or your boss.

Aikido in the Wikipedia.
Jeremy writes advice on choosing a martial arts school.