"I did not create the Jinn, nor mankind, except to worship Me."
Shaykh ul-Islam Ibn Taymiyyah (d. 728H) - rahimahullah - said:
"Worship (Al-'Ibadah) is obedience to Allah by following that which He ordered upon the tongues of His Messengers."
He also said:
"Worship (Al-'Ibadah) is a comprehensive term covering everything that Allah loves and is pleased with - whether saying, or actions, outward and inward."
Ibn Al-Qayyim (d. 751H) - rahimahullah - said:
"Worship revolves around fifteen principles. Whosoever completes them has completed the stages of'ubudiyyah (servitude to Allah). The explanation of this is that 'ibadah is divided between the heart, the tongue, and the limbs. And that for each one of these three come five types of rulings, covering all actions:wajib (obligatory), mustahabb (recommended), haram (prohibited), makruh (disliked), and mubah(permissible)."
Imam Al-Qurtubi (d. 671H) - rahimahullah - said:
"The root of 'ibadah is humility and submissiveness. The various duties that have been prescribed upon the people are called 'ibadat (acts of worship), since what is required is that these acts of worship must be done with humility and submissiveness to Allah - the Most High."
Ibn Kathir (d. 774H) - rahimahullah - said:
"And 'ibadah is obedience to Allah by acting upon what He commands, and abandoning what He forbids; and this is the reality and essence of Islam. And the meaning of Islam is: istislam (submission and surrender) to Allah - the Most High - along with the utmost compliance, humility, and submissiveness to Him."
He also said whilst explaining the above ayah:
"Indeed Allah - the Most High - created the creation so that they could worship Him alone, without associating any partner with Him. Whoever obeys Him will be completely rewarded, whereas whoever disobeys Him would be punished with a severe punishment. And He has informed that He is neither dependant, nor does He have any need for them. Rather, it is they who are in dire need of Him, in every condition and circumstance, since He is the One who created, sustains, and provides for them."
The Meaning of Taghût
The statement of Allah - the Most High:
"And to every nation We sent Messengers, ordering them that they should worship Allah alone, obey Him, and make their worship purely for Him; and that they should avoid At-Taghut."
And at-taghut is from the word at-tughyan, which means: going beyond the limits.
'Umar ibn Al-Khattab radhiallahu 'anhu said:
"The taghut is Shaytan."
Jabir radhiallahu 'anhu said:
"[They] are the soothsayers and fortune-tellers upon whom the devils descend."
Imam Malik (d. 179H) - rahimahullah - said:
"Taghut is that which is worshipped besides Allah."
So this is what has been mentioned by certain people. However, Imam Ibn al-Qayyim - rahimahullahu ta'ala - has given a very comprehensive definition, so he said:
"The taghut is all that which causes the person to exceed the limits with regards to that which is worshipped, followed, or obeyed. So the taghut in any nation is whosoever turns to other than Allah and His Messenger for matters of judgement; or is pleased to be worshipped besides Allah; or is followed without a clear proof from Allah; or is obeyed in that which is known to be disobedience to Allah."
(s) Fath ul-Majid li-Sharh Kitab at-Tawhid (1/84-90)