Of the many names for God, Brahman is the most often used in metaphysics.
Brahman, the Source, the One and only, can be apprehended and worshiped in 2 main ways:
As Nirguna Brahman (God ‘without attributes’)
Nirguna Brahman is Absolute Brahman to which neither name nor form can be attributed.
Parama Atma (Infinite Soul) is Nirguna. He is beyond attributes, or any thing known to humans.
As Saguna Brahman (God ‘with attributes’)
Saguna Brahman implies eminence and a visualization of the divine
‘With attributes’ means with a name and form (nama-rupa), so Saguna Brahman would apply to Krishna, Rama, Jesus, the Trimurti or any other deity of choice. Saguna Brahman suggests a visualization in form and name.
For instance, the Earth is a goddess worshiped by almost all cultures in the world. Equally famous is Sun worship. Worship of gods of planets is also famous. All these gods come under Saguna.
Representation of the Trimurti (reproduction of the Trimurti from the Elephanta caves)
It is humanly impossible to explain Shiva, just as it is impossible to explain the universe. From simple human intellect, he is the one from whom the Brahman emerges, in whom it stays and within whom it goes back.
Here Brahman is not mere the Universe but is the one who is the base and cause of the universe. Brahman holds the universe. Shiva is reasons of all causes.
Scriptures call him the Nirgun Brahman and the Sagun Brahman.
Nirgun: In this state he is formless, and the whole universe and creation lie in the pervasiveness of Shiva.
Saguna: In the Saguna state Shiva is the entire universe and his “ansh” is present in the tree, the insect, the animal, male, female and the whole creation. In this state though all forms arise out of him yet, no form can describe him.
Nirgun-Sagun: In the Nirgun-Sagun state Shiva is worshiped as the Shivalingam. The word Shivalingam has been derived from Sanskrit roots Shiva (Lord) + Lingam (Mark/Chinha/Symbol).
Hence, Shivalingam is the mark of lord within its creation. Everything in the world arises out of a dome / a ball / a bindi. Be it a tree which comes from a seed which is round, a child which comes from a cell which is round, all heavenly bodies are round, the minutes of cells are round and our very earth is round.
Since Shiva can’t be comprehended, we worship his mark or we worship the whole Brahman in a Shivalingam.
Thus, it is important to understand that he is “Formless”. Though all forms belong to him and he is happy in whatever way you look at him yet there is no single form in the world that describes him
Source: The Shiva Tribe