Wahdat ul Wajood or Sufi Doctrine of the Unity of Existence means that only Wajood or existence is of Allah and everything else is dependent upon Him. Or they say it is Allah that exists in everything and this whole universe is a mirror of Him. It came out of the belief that only Allah has to live forever and he is independent while every other thing of this world is dependent on Him. As Shah Hussain R.A has said. "Kahay hussain faqeer sayeen da main naahin sabh tun". It can also be defined by the Mansoor Al-Hallaj's Anal Haq
which is thought to be said by him in madness of love with Allah that he said himself God.
Wahdat ul Shahood is opposite of the above doctrine (which basically emerged out of the danger that common people could possibly forget any differentiation between Allah and His creation, which Sufis infact said due to their ultimate love with the Creator and doesnot actually means that Allah and Creation is one, it just meant that Allah and Creation is very close to each other) and it says that Allah and His creation are separate and nothing can be merged with Allah.

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We say unto it:Be. And it is (Quran 16:40)