Ksemaraja, a disciple of Abhinavagupta, opens the commentary on the Shiva Sutras with this statement:
I surrender my body and mind to that supreme God consciousness of Siva, who is the whole universe and who is one in many. It is from Him that the Rudras and the Ksetrajnas arise and it is in Him that they rest.
What is the exact difference between Rudras and Ksetrajnas?
Rudra can have several connotations.
However, all of them trace their origin to the formless - nirguṇa Supreme Śhiva (in Śhaivism). One can read about some detailed classifications from this QnA too.
The concept of Kṣhetra (the field) & Kṣhetrajña (knower of the field), is clearly explained by Lord Kṛiṣhṇa in Chapter 13 of the Śrīmada Bhagavad Gītā:
श्री भगवानुवाच
इदं शरीरं कौन्तेय क्षेत्रमित्यभिधीयते ।
एतद्यो वेत्ति तं प्राहुः क्षेत्रज्ञ इति तद्विदः ।।13.1।।English Translation By Swami Gambirananda
13.1: The Blessed Lord said O son of Kunti, this body is referred to as the
'field' (i.e. Kṣhetra). Those who are versed in this call him who is conscious of it as the 'knower of the field' (i.e. Kṣhetrajña).
And then the Lord declares himself to be the 'knower of the field' (i.e. Kṣhetrajña) across every whichever way it is.
क्षेत्रज्ञं चापि मां विद्धि सर्वक्षेत्रेषु भारत ।
क्षेत्रक्षेत्रज्ञयोर्ज्ञानं यत्तज्ज्ञानं मतं मम ।।13.2।।English Translation By Swami Gambirananda
13.2: And, O scion of the Bharata dynasty, under-stand Me to be the 'Knower of the field' in all the fields. In My opinion, that is Knowledge which is the knowlege of the field and the knower of the field.
Let's understand this concept of Kṣhetra (the field) & Kṣhetrajña, a bit more in light of some relevant commentaries (bhāṣya) by āchāryas on the Gītā:
13.1-2: Somewhere in the scriputures it is heard that 'The Field-sensitizer must be worshipped'. Is He the same as the Soul, or the Lord or an altogether different third entity ? On a doubt regarding this problem - The Bhagavat instructs-Idam etc. Ksetrajnam etc. For persons of wordly life their body is the Ksetra 'Field' where the seeds of action grow. That is why their personal Soul covered with incoming (or foreign, not-natural) dirt, is called Field-sensitizer. In the case of the enlightened persons, the self-same body is [again] the Ksetra, But there is difference in etimological meaning viz : It decays the fetters of the result of action by means of consuming [it]; and it protects [them] from the fear of birth-and-death [cycle]. With reference to these persons, the Supreme Soul, Vasudeva is the Field-sensitizer. He who sensitizes this Field: He who causes it to know. Here the root vid includes within itself, the meaning of the causal suffix ni. Therefore [the meaning is :] He, on account of Whose grace this insentient [body] attains the status of being sentient, He alone, and no one else, is the Field-sensitizer. But, the [only] difference is this: Taking into consideration the aspect of limited pervasiveness, He is taken to be Soul; and on account of [His] unlimited pervasiveness in all the Fields, [He is called] the Supreme Soul, the Bhagavat Vasudeva. Of Me: the Sixth Case here is in the objective sense. Hence the idea is: I may be known by means of this knowledge.
13.1: The Lord specifies the body as the object referred to by the pronoun idam (this). O son of Kunti, (this body) abhidhiyate, is referred to; ksetram iti, as the field-because it is protected (tra) against injury (ksata), or because it perishes (ksi), wastes away (ksar), or because the results of actions get fulfilled in the body as in a field (ksetra). The word iti is used in the sense of 'as'. They-who?-tadvidah, who are versed in this, who know the 'field' and the 'knower of the field'; ahuh, call; tam, him, the knower; yah, who; vetti etat, is concious of, knows, it, the body, the field-makes it, from head to foot, an abject of his knowledge; makes it an object of perception as a separate entity, through knowledg which is spontaneous or is acired through instruction; ksetrajna iti, as the knower of the field. As before, the word iti is used in the sense of 'as'. They call him as the knower of the field. Is it that the field and the knower of the field thus mentioned are to be understood through this much knowledge only? The answer is, no.
13.2: Ca api, and; viddhi, understand; mam, Me, the supreme God who is transcendental; to be the ksetrajnam, 'Knower of the field' with the characterisitics noted above; sarva-ksetresu, in all the fields. The idea is this: Know the 'Knower of the field'- who has become diversified by limiting adjuncts in the form of numerous 'fields' ranging from Brahma to a clump of grass-as free from differentiations resulting from all the limiting adjuncts, and as beyond the range of such words and ideas as existece, nonexistence, etc. O scion of the Bharata dynasty, since there remains nothing to be known apart from the true nature of the field, the knower of the field and God, therefore; tat, that; is jnanam, Knowledge, right knowledge; yat, which; is the jnanam, knowledge; ksetra-ksetrajnayoh, of the field and the knower of the field-which are the two knowables-, and by which Knowledge the field and the knower of the field are made objects of knowledge.
Thus, basically, the Kṣhetrajña refers to the one who is "aware" of the Kṣhetra. For normal humans, in the material sense, it means one who is aware and knows about his body & mind. Thus, we may also understand by a little contemplation that we are different from the body. I or you or anyone else who owns the body is called kṣetra-jña, the knower of the field of activities, and the body is called kṣetra, the field of activities itself. For the yogīs or jñanīs: i.e., the enlightened being - it's the knowledge of the Self (ātman) viz. a viz. the Supreme Self (paramātman). Because, a living "conditioned soul" can thus understand that he is different from the body. And finally, the Lord (Supreme Brahman) being the effective cause of everything, and also encompasses everything, and thus the Lord themselves is to be called as 'the Ultimate field (Kṣhetra)', thus, Para-Brahman (Śhiva/Kṛiṣhṇa-Viṣhṇu/Devī/Saguṇa Brahman) is the Ultimate 'knower of the field' (Kṣhetrajña).
Rudras are a special class of "Śhiva-tattva" entities/deities, varying from 1 (Shiva-Rudra of Kailāśha), 8 (AṣhṭaMūrti Rudras), 11 (the main eleven Rudras), 20 to 60 (in their reference with the Maruts in the Vedas) to practically infinite, pervading all forms of creation/existence everywhere, in their myriad forms.
Kṣhetrajña itself can mean:
the Supreme Brahman (in this question specifically - Śhiva himself),
Or it can mean all the enlightened/ exalted beings (may include various forms of the Saguṇa Brahman, jñanīs, yogīs, et al),
or the normal "jīva" too, who is in the material creation, trying to be aware of itself.
EDIT
In the light of Kashmiri Shaivism (Trika Shaivism) - What is the exact difference between Rudras and Kṣetrajñas?
- Kṣetrajña: is the soul in bondage, known as paṣu, full of Āṇavamala (आणवमल)ξ (inherent limitation) from Brahmā down to the tiniest creature.
- Rudras: are the free souls whose anava mala has completely vanished. They are in the category of patiΨ. They bring about Sṛṣṭi (emergence), Sthiti (maintenance), and Saṃhāra (withdrawal) of the world system according to the karmas of individuals. Śiva is directly concerned only with vilaya (veiling of the essential nature of the soul) and anugraha (grace or unveiling of the essential nature). Ananta Bhattaraka, Aghoresa, and Rudra are synonyms. Rudra corresponds to Iśvara of Śankara's Advaita Vedanta.
ξ: Anava mala (Ajñana: one of the bondages in Trika, 'mala' literally means taint or impurity) is an innate, limiting condition which is the primal ignorance of our essential nature as Śiva. The bondage of the individual is due to 'innate ignorance' or 'anava mala'. It is this primary limiting condition that reduces the universal consciousness to an 'anu' or a limited creature. It comes about by the limitation of the iccha Sakti of the Supreme. It is owing to this that the jiva considers himself to be a separate entity cut off from the universal stream of consciousness. It is the consciousness of self-limitation and imperfection or it is only vitiated knowledge by which one considers the vehicles - gross, subtle bodies, etc. as the Self. Anava mala is of two kinds - (1) Paurusa and (2) bauddha.
Ψ: Pati: The experient of Suddha-adhva (adhva literally means course or path); i.e., the liberated individual.
Quoting from - Siva Sutras: The Yoga of Supreme Identity authored by Jaideva Singh,
Publisher: Motilal Banarsidass Publishers Pvt Ltd. [2006 Reprint Edition]
