The term Hindu is exogamous (given by outsiders), while Sanātana-dharma is a term that was internally derived. Importantly, all Hindus practice Dharma, which is a cognitive state of stability in any situation. Since it is a cognitive state, it manifests as experience, which can come only from practice of awareness (prajñā) which starts at birth and stays for a lifetime. 

Materially, Dharma is derived from the Veda which are considered the source of all knowledge or āstika, and also apaurusheya (not of a person, meaning, the knowledge comes when the person was in a state of liberation).

Dharma splits life into four reasons for existence or Puruṣārtha, which are artha (material element), kāma (sensual element), dharma (stability and order) and finally mokṣa (liberation). Puruṣārtha change as one progresses in life, called  āśrama or stages of life. There are four āśrama, and these are brahmacaryāśrama or studentship, gṛhasthāśrama or house-holders or,  vāṇaprasthāśrama or retirement, and sannyāsāśrama or the state of liberation or mokṣa.

Awareness or prajñā also changes depending on one’s work or vārna. There are four employment states – brāhmana (one who practices brahma-vidya or understands the nature of the Brahman), kṣatriya (warrior or keeper of dharma), vaiśya (businessman) and śūdra (worker).

Life is split into sixteen gateways or refining stages and these are called samskāra. However, since Dharma or stability is also dependent on the place (sthala), time (kāla) and cuisine (pātra), different people and families develop their own practices and these are called sampradāya.

The rules governing all of the above states are called śāstra or compendium of rules.

However, to ensure that people practicing Dharma do not get mired in chaos, everyone is allowed to practice a personal dharma while adhering to the rules of yama (interacting with the environment) and niyama (self-control).

guru is a combination word – gu = darkness + ru = light. So, guru is a person who guides a student from light to darkness.

Guru also translates to a weighty one. So, a Guru should not only have the technical competence or subject matter expertise, he or she must also have the internal capability to understand and  guide the student through all the challenges the student may face. This calls for enhanced awareness of the Self (prajñā), and a willingness to put the student’s interest before oneself.

Therefore, the state of a Guru is total commitment, this means that for a Guru, the student or siśya becomes an extension of himself or herself. This means a cognitive as well as intellectual integration between the Guru and student (siśya).

Importantly, the student surrenders completely to the Guru, this means that he or she ceases to exist as a person, but becomes a tool in the hands of his or her Guru, to be pounded and pestle into the person the Guru deems him fit to be.

Consequently, the student cannot judge the Guru’s action. He can only accept without questioning.

This makes the Guru-siśya relationship a unique one, almost like a rebirth.

No single text discusses or explains the many philosophies which fall under the umbrella of sanātana-dharma. 

Sanātana-dharma philosophy encompasses multiple schools of thought or shākhās which are subdivided into sub-schools or pravara. Further, these get subdivided depending on interpretation, region, culture, practice and most importantly, guru.
Learn the fundamentals about Sanatana Dharma