In the Yoga Sutra's written by Sage Patanjali he shares his wisdom gained through experience and discusses those things that are important and necessary for the spiritual aspirant to achieve in order to reach that final goal.  The 195 Sutras of Patanjali are divided into four chapters:

1. Samadhi-Pat (Samadhi- the highest state of consciousness)
2. Sadhana-Pat (Sadhana- spiritual practice)
3. Vibhuti-Pat (Vibhuti- power, siddhi)
4. Kaivalya-Pat (Kaivalya- unity with the Absolute)

"Pat" means Chapter, as well as a "level" or "state" of consciousness.

SAMADHI-PAT

Before someone decides to follow a certain path, it is good to know the aim of that path.  This is the reason why Patanjali, at the beginning of his Sutras, speaks about the highest aim of Yoga - Samadhi, that state in which the Knowledge, the Knower and the object of Knowledge become one.  Or, as can otherwise be said, that state where there is Unity with the Highest-Self.  There are three definitons of the "SELF":

BRAHMAN or PARAMATMA, the highest Divine Self which is without any attribute or qualities - it is omnipresent and ever-lasting.

ATMAN, the cosmic soul, which is also immortal yet limited through its bondage with the three Gunas and attributes.  This bondage divides it from the Highest-Self.  The Atman manifests as the third definition of the "Self",

JIVATMA, the individual Soul.  Jiva means "living being", and is also known as the reflection of Atma.

It is possible for people to reach the highest state of Samadhi - union with the Highest-Self - through the help of Yoga exercises and Meditation. There are many different levels of Samadhi, but the three main ones are:

BHAVA SAMADHI
SABIJA or, SAVIKALPA SAMADHI
NIRBIJA or, NIRVIKALPA SAMADHI

BHAVA SAMADHI is similar to a trance state.  One can experience this samadhi during prayer or in a moment of deep spiritual emotion. However, this type of samadhi may sometimes present as an obstacle upon the path, as there can be a tendency to remain absorbed in the experience of beautiful feelings, and this impedes further effort. It is a misunderstanding and great mistake to believe that one can attain Bhava Samadhi through the use of drugs or alcohol.  One may for a moment feel happy and free, and forget about the outer reality with its associated problems, but  the comeback from this artifical "high" state of drugs is extremely painful, not to mention that these substances also create physical and psychological problems.  Real Bhava Samadhi can only be attained through Bhakti and devotion to God, either through prayer or meditation.  However, even though this state brings wonderful spiritual experiences, it is not the ultimate aim.

SABIJA SAMADHI is the next level of samadhi that can only be reached in deep meditation.  'Bija' means seed, and here it is used in the sense of the "cause" of karmas. Sabija Samadhi is a state of samadhi where the "seeds" of karmas are still present, ie. wishes and desires are still within the aspirant.  Under appropriate circumstances a mighty tree can grow from just one seed.  And so are our wishes hidden in the subconscious waiting for the occasion to rise to the surface and again bear new fruit - i.e. new karmas.  Therefore, Patanjali says right at the beginning of his Sutras, that we should strive towards finding these seeds that are within and dissolve them or "burn" them, as seeds roasted by fire cannot sprout anymore.

In the second Sutra of the first Chapter, it is said:

YOGA CHITTA VRITTI NIRODHAH.
"Your Yoga path begins after you have stopped, or brought to a stand-still the Chitta Vrittis".

CHITTA means consciousness and VRITTIS means waves. When water has been disturbed by waves, we can no longer see the bottom and we are unable to recognise the pearl that is lying there.  But the diver who would like to retrieve the pearl uses a special method - he lays a glass-shield upon the water's surface and breaks the waves that way. Now he can see the pearl and bring it up.  And so we also should find that window, that appropriate technique through which we can halt and control our Vrittis, so we can then merge into the peace within and find those inner pearls and qualities.  Waves only appear on the surface, but within its depths the ocean is completely peaceful.  A Yogi must establish that peace firmly within, just like the ocean which reigns supreme peace within its depths.  Whoever is disturbed by inner and outer  restlessness will very soon show signs of physical and psychic exhaustion, or other symptoms of illness.

Once all the hidden seeds of subconscious desires (which are the origin of new karmas), are found and dissolved, the Yogi will finally enter the highest state of Samadhi - Nirbija, or Nirvikalpa Samadhi. That Yogi now finds himself at the end of his search.  This connection of the Knower (who is yourself), and the Object of Knowledge (also yourself), will be established through Knowledge.  As long as Knowledge is missing there is no union possible and the Knower will feel separated from the Object of his Knowledge.  Or as can otherwise be said: as long as Knowledge of the Self is missing, no Self-Knowledge is possible.

Here is an analogy:  When two people who are living together fight and are in disharmony, when there is misunderstanding between them, this means the right knowledge is absent.  If understanding is awakened on both sides through a certain event, for example by talking to each other and giving an explanation, then the fighting will come to an end. In Yoga, this event that brings right knowledge is our practice or Sadhana.  As long as we continue to live in ignorance (ie. without the right Knowledge), then we will always suffer.  Only Knowledge will free us from that suffering.

YOGA means to follow a firm discipline.

Patanjali explains these principles in the first part of his Sutras - Samadhi-Pat.  In the first Sutra he says: 

ATHA YOGA ANUSASANAM.

The student asks the Master, "Master, I would like to practise Yoga. What should I do ?"  And the Master answers, "I will now explain to you the meaning of Yoga", and thus he begins to teach the student.

YOGA means to follow a firm discipline.  If the student is unable to follow this discipline, then he will not succeed, even after many years of effort.  What then are the most important prerequisites on the path of Yoga ?  The most important is that the student be a real aspirant, one who has made a firm decision to follow this path and to realise the aim.  Difficulties, which are always present, will never persuade the real aspirant to change his decision, he always remains steadfast on the path and doesn't change his view or feelings.  The butterfly flutters from blossom to blossom, but the nature of a true Yogi is firmness and stability.  The next important prerequisite is practice. "Only through practice can one become a Master".  Without regular practice, nothing can be realised, there will be no success.  One can assure his or her friend a hundred times on the telephone "I would really like to see you so much", but if that person doesn't get going to see their friend the affirmations are useless.  No-one can have children just by reading books about conception and birth.  In the same way, it is impossible to attain the final goal just by reading sacred scriptures - only practice can bring Realisation.

How can our practice be successful?  Patanjali explains in Sadhana-Pat.