Here is my favorite explanation of the concept of Ahimsa - it comes from a conversation between Gandhiji and Dr. Howard Thurman, an African-American minister and writer:
...without a direct active expression of it, non-violence to my mind is meaningless. It is the greatest and the activist force in the world. One cannot be passively non-violent. In fact 'non-violence' is a term I had to coin in order to bring out the root meaning of Ahimsa. In spite of the negative particle 'non', it is no negative force. Superficially we are surrounded in life by strife and bloodshed, life living upon life. But some great seer, who ages ago penetrated the centre of truth, said: It is not through strife and violence but through non-violence that man [sic] can fulfil his destiny and his duty to his fellow creatures. It is a force which is more positive than electricity, and more powerful than even ether. At the centre [sic] of non-violence is a force which is self-acting. Ahimsa means 'love' in the Pauline sense, and yet something more than the 'love' defined by St. Paul, although I know St. Paul's beautiful definition is good enough for all practical purposes. Ahimsa includes the whole creation, and not only human. Besides 'love' in the English language has other connotations, and so I was compelled to use the negative word. But it does not, as I have told you, express a negative force, but a force superior to all the forces put together. One person who can express Ahimsa in life exercises a force superior to all the forces of brutality.