"Truly, your god is truly One( Allah )!" [Qur'an 37:4]
Tawheed means unification (oneness). It is the greatest foundation of Islamic belief. In fact, a person is considered as Muslim if he/she accepts that there is no god worthy of worship but Allah and that Muhammad is His slave and His messenger to humankind.
Traditionally, Tawheed has been divided into three categories which help us to understand why Allah alone deserves to be singled out for worship
1. Tawheed ar-Ruboobeeyah ("Maintaining the Unity of Lordship")
This first category of Tawheed is known as Tawheed-Ar Ruboobiyyah or the unity of Allaah's Lordship. Through this we understand that it is Allaah 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 Allaah Alone Who controls the universe and allows things to happen.
2. Tawheed al-Asmaa was-Sifaat ("Maintaining the Unity of Allah's Names and
Attributes")
This category of Tawheed is known as Tawheed Al-Asmaa' Was-Siffaat or the unity of Allaah's Names and Attributes. As Allah said:
وَلِلَّهِ ٱلۡأَسۡمَآءُ ٱلۡحُسۡنَىٰ فَٱدۡعُوهُ بِہَاۖ وَذَرُواْ ٱلَّذِينَ يُلۡحِدُونَ فِىٓ أَسۡمَـٰٓٮِٕهِۦۚ سَيُجۡزَوۡنَ مَا كَانُواْ يَعۡمَلُونَ And (all) the Most Beautiful Names belong to Allâh[], so call on Him by them, and leave the company of those who belie or deny (or utter impious speech against) His Names. They will be requited for what they used to do. (Surah Al 3araf:180)
3. Tawheed al-'Ebaadah/ Ulooheeyah ("Maintaining the Unity of Allah's Worship")
This part of Tawheed is known as Tawheed Al-'Ibaadah, or the unity of Allaah'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.
"And your Lord says, Call upon Me; I will respond to you.” [Quran 40:60]
The opposite of Tawheed is 'Shirk' which is to associate partners with Allah by giving (or attributing) that which is exclusively His to others.
"Surely Allaah will not forgive the association of partners (Shirk) with Him, but He forgives (sins) less than that of whomever He wishes."( Soorah an-Nisaa, 4:48.)
Two types of shirk: major shirk and minor shirk
1 , Major shirk is devoting any kind of act of worship which should be for Allaah to someone other than, such as praying to anyone other than Allaah, fasting for anyone other than Allaah or offering a sacrifice to anyone other than Allaah. It is also a form of major shirk to offer supplication (du’aa) to anyone other than Allaah, such as calling upon the occupant of a grave or calling upon one who is absent to help one in some way in which no one is able to help except Allaah.
2. Minor shirk, which means every kind of speech or action that Islaam describes as shirk, but it does not put a person beyond the pale of Islaam – such as swearing an oath by something other than Allaah, because the Prophet (sal-Allaahu ‘alayhe wa sallam) said,
( “Whoever swears an oath by something other than Allaah is guilty of kufr or shirk.” )
It is not permissible to swear by the Prophet (sal-Allaahu ‘alayhe wa sallam), or by the president, or by the Ka’bah, or by Jibreel, because this is shirk, but it is minor shirk which does not put a person beyond the pale of Islaam.
Another type of minor shirk is showing off( Raya ), which means that a person does something so that people will see it, not for the sake of Allaah.
The ways in which showing off may cancel out acts of worship are either of the following