Hindustani music has main 7 स्वर (swar):

• षड्ज (Shadaj): pronounced सा (Sā)
• ऋषभ (Rishabha): pronounced रे (Re)
• गान्धार (Gāndhāra): pronounced ग (Ga)
• मध्यम (Madhyam): pronounced म (Ma)
• पंचम (Paṅcam): pronounced प (Pa)
• धैवत (Dhaivat): pronounced ध (Dha)
• निषाद (Nishād): pronounced नि (Ni)

These notes from the classical texts but are also referred to by their corresponding solfege syllables, the entire group being called सप्तक (Saptak). ^*Sapt = 7 [Solfege is a system used to teach pitch and sight-singing in music. It assigns specific syllables to each note of a scale, which helps musicians to learn and recognize pitches, intervals, and melodies. Solfege is widely used in music education, especially for vocal training, and is a foundational tool for understanding melody and harmony.]
The eighth note is a repetition of the first thus making an octave between the two with a ratio of 1:2 and forming a consonance.

You'll see that between the Sa-Re, Re-Ga, Ma-Pa, Pa-Dha, and Dha-Ni there is another key that lies in between each pair but notes Ga-Ma and Ni-Sᵒa. The former notes are a whole step (tone) apart while the latter are semi-tone apart. The former is called the शुद्ध स्वर (Shuddha Swar) i.e. pure, natural notes. The intermediary notes in the series Sa-Re, Re-Ga, Ma-Pa, Pa-Dha, and Dha-Ni can be denoted as Ra, Ga, Mâ, Pa, Dha, and Ni. are called विकृत स्वर (Vikrit Swar) or altered notes. There is however no notes between Ga-Ma and Ni-Sa as these notes are musically too close to allow any interpolation and would be difficult to make out any स्वर (Swar) between them.

So far we have come across the terms नाद (Nāda), स्वर (Swar), and श्रुति (Śruti) to refer to the musical sounds but what makes them different from each other and where do these terms fit in the musical scale?

1. स्वर in Saptak (octave): There are the 12 different (7 Shuddha and 5 Vikrit) types of musical notes identifiable by the ears in a Saptak. However 10 of these notes excluding Shadaj (the tonic) and Pancham (the third harmonic) actually have a range of or a group of individual frequencies which are all identified as the same Swar (note). This range should actually be called स्वरक्षेत्र (Swarakshetra). [needs more info on this...]
2. स्वर in Rāga: These are the precise single frequencies within a Swarakshetra which make the scale of the Rāga. [more on Rāga in later articles]

There are innumerable musical sounds as individual frequencies called Nāda within 2 Shadaj (i.e. an octave or Saptak). In order to create Hindustani music (i.e. Rāga) we must be able to "stay" on some of them. These selected Nāda to be used to stay on are called Śruti and there 22 of them [more on why they are 22 will be in late articles]. Out of these 22 the ones selected for Rāga becomes Swar of that Rāga. Thus,

• All individual frequencies between 2 Shadaj are Nāda
• Those Nāda selected to stay on in Rāga become Śruti
• The selcted Śrutis in a Rāga are Swar.

Śruti comes from an old Sanskrit saying श्रु्यते इति श्रुति (śrūyate iti śruti) which means when a Nāda is heard so clearly that it can be 'identified' as it is called a śruti. This is possible only when we stay on a Nāda which allows our ears to recognise it as a musical note.
Consider this analogy: All the players in Cricket are Nāda, the 22 selcted are Śruti and the ones playing for a particular match are Swar.

Question: why do we stay on only the 22 Śruti? is it due to their relationship with Shadaj? (research this)