Arnica comes in many different forms. I personally like to use the balm, and gently massage it in to warm and soften the muscles, and improve fluid drainage. It can be taken orally in tablet form, but our gut is constantly digesting and absorbing throughout the day, so its nice to let the skin do some work too. Studies have shown Arnica to reduce swelling, pain and have an antibiotic effect.
Perhaps most commonly Arnica is used to reduce bruising. It might be taken or used before/after an operation, or to diminish general day-to-day bruises. Patients taking blood-thinners such as Warfarin are more susceptible to bruising, and those of us who are accident-prone!!
Arnica may be used before or after exercise to reduce muscle aches and pains, or to support a tired body. I have recently been using it on a calf strain, and found it to be even better than magnesium spray! You can use Arnica instead of massage oil and it smells great - take some with you to your therapist if they don't mind using it.
Some research has found Arnica to be useful for those suffering from wear-and-tear related pain. For example, rub it in to stiff and painful hands or knees. This should be tried for a minimum of 3 weeks, applying arnica twice daily.