Linguistically, the word Tawheed, in Arabic, means unification (to make something one). Islamically, it is in reference to Allah being singled out alone, in all that is particular to Him. The opposite of Tawheed is 'Shirk' which is to associate partners with Allah by giving (or attributing) that which is exclusively His to others.
Traditionally, Tawheed has been divided into three categories which help us to understand why Allah alone deserves to be singled out for worship. The division of Tawheed into these three categories is something which was not done by Prophet Muhammad or any of his companions, may Allah be pleased with them. So, is this Bid'ah (i.e., something new that has been introduced into the religion)? The answer to this is no, because we find that the basis of these three categories are to be found in the verses of the Quran, Hadeeth [authenticated Prophetic reports] of the Prophet and the statements of his companions.
The necessity for dividing up Tawheed into three categories first came about during the early days of Islam. After the death of the Prophet Islam spread like wildfire and before long the empires of Africa, Byzantium Persia and India all fell under Muslim rule. Many people from these areas became Muslims. However some of these converts to Islam also carried with them some of their old pagan beliefs. This caused much confusion amongst people, and the simple belief of Islam which the people had accepted started to become clouded. The task of opposing these new thoughts and ideas which were becoming prevalent, fell upon the shoulders of the Muslim scholars, who rose to meet this challenge intellectually. Amongst the first people to divide Tawheed into three categories was the famous scholar of Iraq Abu Haneefah .
Before mentioning what these three categories are, it has to be stressed that the concept of Tawheed is in no way similar to the Christian concept of trinity. The division of Tawheed does not divide Allah up into three separate parts (as does trinity), but rather it helps us to understand as to how our Creator is Unique and Alone in being singled out for worship and reverence. The three categories of Tawheed are:
1- The Unity of Allah's Lordship
This first category of Tawheed is known as Tawheed-Ar Ruboobiyyah or the unity of Allah's Lordship. Through this we understand that it is Allah Who Alone created the universe, which is why one of His divine names is Al-Khaaliq (the Creator). Through this we know that it is Allah Alone Who controls the universe and allows things to happen. This is why Allah refers to Himself in the Quran as Al-Malik (The Sovereign). Hence, when something happens, it only happens with the permission of Allah, as stated in the Quranic verse (which means): {And no calamity strikes except with the permission of Allah} [Quran 64:11]
The Prophet further elaborated on this concept of Allah's control over the universe by saying: "Be aware that if the whole of mankind gathered together in order to do something to help you they would only be able to do something for you which Allah had already written for you. Likewise, if the whole of mankind gathered together to harm you, they would only be able to do something to harm you which Allah had already written to happen to you". [At-Tirmithi]
We know that another name, which Allah chooses to call Himself by, is Ar-Razzaaq (the Sustainer). It is Allah who provides us with our food, shelter, clothing, families and friends. When our crops seem to be dying and there is not a cloud in sight, it is Allah Who sends down His rain from the skies and sustains us. Allah Says (what means): {Allah is the Creator of all things and He is, over all things, Disposer of affairs.} [Quran 39:62]
From this you would expect that people would turn back to Allah for good or bad fortune. However, this is not the case. Today we find some people relying on all kinds of good luck charms, which they believe will bring them good fortune and ward off evil.
These practices have absolutely no basis whatsoever in Islam, but rather the Prophet warned us by saying: "Whosoever brings something new into Islam (which does not belong to it) will have it rejected". [Al-Bukhari]
2- The Unity of Allah's names and Attributes
This category of Tawheed is known as Tawheed Al-Asmaa' Was-Siffaat or the unity of Allah's Names and Attributes. Allah Says in the Quran (what means): {Allah — there is no deity except Him. To Him belong the best names.} [Quran 20:8]
This category of Tawheed helps us to understand Who our Creator is through His Names and Attributes. Through His Names and Attributes we know that Allah is far beyond our imagination and bears no resemblance to human beings. In fact, it is this principle which makes Islam unique from all the other religions on the face of this earth. Allah Says (what means): {There is nothing like unto Him and He is the Hearing, the Seeing.} [Quran 42:11]
3- The Unity of Allah's Worship
This part of Tawheed is known as Tawheed Al-'Ibaadah, or the unity of Allah's worship. It is this aspect of Tawheed which is the most important. It is through this that we learn how to worship our Creator alone. Allah is not the kind of god who does not respond or hear your calls. He is not the kind of god who needs some sort of middleman to take our prayers up to Him. Rather, Allah says (what means): {And your Lord says, Call upon Me; I will respond to you.} [Quran 40:60]
There is nothing to prevent a person from calling directly upon Allah and asking for His help and forgiveness. Unfortunately, this is violated by most of mankind who believe that other men can intercede for them and even grant them forgiveness because of their apparent special status! An example of this is the Catholic religion which holds that the celibate priests are purer than normal people. This allows for them to then hear the confessions of people's sins and subsequently grant them forgiveness. It is this kind of behaviour which takes a person away from the worship of Allah to the worship of man.
As Muslims, we too should be careful of this, because it seems that many of our brothers and sisters are indulging in this kind of evil and are not even aware of it. How many Muslims are there today who call upon others besides Allah? We find that in some countries, there are millions of Muslims who go to the graves of 'saints' and ask them for children, wealth, fame and more sadly, forgiveness. What makes this more ironic is the fact that many of these Muslims pray five times a day, and in every Rak'ah (unit) of their prayer they say the following to their Creator: {It is You we worship and You we ask for help.} [Quran 1:4]
Learning about the Tawheed of Allah is the most important thing for the whole of humanity. For if we learn how to trust and worship our Creator with sincerity, then -and only then- will we escape from the slavery and captivity of this life. It is by worshipping Allah alone does a man achieve true peace and success.