Okay, briefly, cutting is a coping mechanism, though not necessarily a good one. I do not condone the use of cutting to solve problems despite my own history with it. Mary of Nazareth your arm was inspiring, it the way mine would if I hadn't given my knife away.
Cutters do not often want attention and if you have a friend who does cut for attention, beat the shit out of them because it can become addictive. Medically cutting releases endorphins making a person feel better and certain cutters enjoy the sight (and personally taste) of their own blood. Cutters often cover up their cuts with anything from long sleeve shirts to shea butter to reduce scarring.
Cutters use, but are not limited to (don't underestimate a person's ingeniunitey when they need to satisfy a craving): razor blades, exacto knives, scissors, jack knifes, switch blades,.. saws of various types, and glass.
If you know someone who is a cutter, well don't follow any set advice, your friend may not be suicidal at all, in fact they likely aren't. For those of you who have addicted friends trying to quit, watch them closely because they usually have the incredibly strong urge to make extreme lacerations. Personally, as a cutter trying to quit my withdrawal often leads me to wanting to use suicide grade cuts.
Cutters are not all emo fuck ups, we're normal people too, and some of the nicer ones at that.
I gave my knife away to a friend leaving me with 9 exacto knife blades, a few saws and an array of kitchen knives with which I may cut myself. Cutters use cutting as an escape mechanism