In the Army what is the difference between Corporal and a specialist?

I am a Former Marine and I was wondering what the difference is. In the Marine Corps all E-4s are Corporals no matter what your MOS is. I thought all E-4s where NCO's but I was told that a specialist is not an NCO and also that there are something like 4 different cles of specialist. It sounds confusing could someone please take some time to explain Thank-you.