Self Harm can best be described as "The act of doing anything deliberately harmful to yourself for a reason". This is left fairly generic for good reason - the number of methods, reasons, causes and effects found within the subject of Self Harm are almost infinite.
Self Injury is certainly the most commonly associated 'action' related to Self Harm - indeed the two are often referred to as one and the same. Self Injurious behavior spans the use of cutting, burning, self-poisoning (including OD's), wound interference, bruising, even bone breaking, as well as many more. The core element to Self Injury is endorphin release (other than self-poisoning) - which can lead to a level of addiction to it.