“Yogaschitta Vrittinirodha" - Patanjali
(“Cesssation of the mental modifications is yoga”)

Yoga literally translates to to-join (or) to-unite. It inherently means to unite the jeeva-atma (individual soul) to parama-atma (total consciousness). This union is termed as enlightenment which leads to moksha (a.k.a.) mukti (liberation). Liberation of this individual soul from the cycle of physical birth and death.

To a modern mind it sounds mythical as science couldn't navigate this subject and produce the post-mortem reports. It is purely experiential. When we take a deep-breath (sukha purvaka) and exhale, there is a great sense of pleasantness. Ignore all the sanskrit jargons, prophecies, beliefs and doubts and consider a multitude of this pleasantness is yoga.

Pretty much everything in india roots back to Lord Shiva. Yoga is not an exception. But all the knowledge of the current times comes from the artifacts from two prominent people. Patanjali (allegedly, 400 AD) compiled yoga sutras and svātmārāma (15th century) authored hatha yoga pradipika.

Yoga emphasis on these 8 steps (ashta rung) of the ladder. 
  1. Yama (ethics)
  2. Niyama (self-discipline)
  3. Asana (postures)
  4. Pranayama (mastery over breath)
  5. Pratyahara (mastery over senses)
  6. Dharana (concentration)
  7. Dyana (meditation)
  8. Samadhi (absolute stillness)
Often, yama and niyama are taken for granted as everyone believes they are ethical and disciplined in their own measure. The 5 attributes of each is explicitly listed by patanjali and svātmārāma extended it to 10 yamas and 10 niyamas.

In the western world, Yoga often translates to hatha yoga, which is mainly asana and pranayama (along with kriyas, bandhas, mudras). Svātmārāma's hatha yoga pradipika serves as the ultimate source of truth for the practitioners and pretty much every single commercial yoga school teach a subset of hatha yoga.

The latter half (5-8) is quite advanced and often applicable for the dedicated practitioners.

Pratyahara is complete explicit control over these 10 senses. 
autonomous senses cognitive senses
elimination hearing
procreation tasting
motion (legs) seeing
holding (hands) touching
speaking hearing
Dharana (Concentration) is fixing the attention of mind onto one object or place. 

The Last two (7 & 8), are not something that can be practiced. Rather a flowering of the previous 6 steps (sadhanas) and transcends on the practitioner.

Dhyana (Meditation) is sustained concentration, whereby the attention continues to hold the same object or place without any effort.

Samadhi is total absorption where the practitioner is not identified with their body or mind. It is a complete stillness, total emptiness. Mind is totally tamed and no ripples of thought exists. This state is the goal of all yogic practices.

As obvious as it is, these abilities will enhance anyone to live the life to the fullest. Everything they do will be at its best and automatically the person gets in tune with the cosmos.

As the topic is vast, this could be a 10 part series, uncovering every topic around yoga in detail.