Yoga is a Sanskrit word. It may be originated from the same root of "yoke" -the latin word meaning to join together. Perhaps the best translation of it would be "union". 
But union of what-- that depends on your thinking-whether practical, scientific, religious, or spiritual is it. Here are some the common "unions" that people desire for:- 
Union of body, mind, and spirit 
Union of your "everyday nature" with your "true nature" 
Union of the individual soul and the Divine 
Union of conscious and unconscious mind (or bringing the unconscious into awareness) 
Union of "lower self" and "higher self" 
So Yoga is "union". A sage named Patanjalli who codified many thousands of years ago Yoga, in his work calling "Yoga Sutras" he says, 
"Yoga is the control of the vrittis of the mind". 
The vrittis are waves or modifications such as thoughts, memories, and emotions. But these waves are bad or evil, but just as the waves of a lake reflect the light and keep you from seeing your reflection, the vrittis keep you from seeing your true self. He goes on to say," When the vrittis are controlled, the seer abides in his own true nature". 
Everything has a "true nature", that essence that comes forth when you just leave things alone in their natural environment. 
So, what is the true nature of a human being - your true nature? Someone searching for this, someone trying to be one with his or her own true nature-he or she is a yogi (male) or yogini (female). Since time immemorial the yogis, and sages of all tradition, say that your true nature is peace, bliss, and happiness. When you achieve these you are truly divine. There are many paths for yoga such as -- meditation, praying, contemplation. But you should honor every paths while follow one only religiously.

Astanga Yoga is a systematic branch of yoga consisting eight main parts to it. A sage named Patanjalli about 2000 years ago first wrote it down. Astanga Yoga is often called "Raja Yoga", which means the "Royal" Yoga. This name was actually coined at the turn of the century by Swami Vivekannanda, who thought Westerners would have a hard time saying "Astanga"! 

The eight steps of Astanga Yoga are: 
Yama, or Controls or Restraints 
Ahmisa - living to love and serve No-violence, no-injury to others.
Satya - Truthfullenss, eliminating falsehood from our lives. 
Asteya - Non-stealing, not taking that which is not ours. 
Bramacharya - No-indulgence, keeping the harmony and equilibrium of our lives in tact. 
Aparigraha - Non-possesiveness, yielding, letting go of unnecessary objects which obstruct our progress.

Niyama, or Observances or Actions 
Shaucha - Purifying body, mind, and spirit 
Samtosha - Contentment, cultivating the inner joys generating with freedom from desire 
Tapas - Self-discipline, developing a strong will, training the habits of body and mind 
Svadhyaya - Self-study 
Ishwara Prandihana - Self-surrender to our higher consciousness, our true inner self, and/or to god (notice that yoga acknowledges that there is a god, but does not name any particular god or prescribe particular religious practices). 

Asana or Posture, Pranayama(Control Breath),Pratyahara (Control of the Senses)  Concentration, Dhyana or Meditation,Samhadi or Self-Realization.

The two words –HA and THA –constitute the whole word—Hatha. Ha means The Sun and Tha means The moon. So hatha yoga signifies "union of the sun and moon"i.e. to bringing our light and dark sides, our masculine (strength) and feminine (gentle) sides, our constant nature and our changing nature together. Hatha Yoga includes Yama, Niyama, Asana, and Pranayama. Hatha Yoga is seen as a preparation for the higher steps of Astanga Yoga. It makes the body become comfortable and steady so that it is not an obstacle in achieving the subtler states that meditation makes access able.

What is Meditation?
Dhyana,or Meditation is to keep and concentrate the mind on one point. In this state our awareness cast our everyday awareness aside.

What is Samhadi?
Samhadi,or Self-Realization, is the last step of Astanga Yoga. It is a transcendence of the mind, going beyond the normal limits of what we normally call our self or ego. It is becoming one with the substance and flow of creation. It is like the feeling when we become totally lost in something outside of ourselves, perhaps when watching a beautiful sunset or waterfall, being absorbed in a game or story, or watching your children experience joy.

What is Kundalini Yoga?
Kundalini Yoga is considered the mother of all the Styles of Yoga. It works on awakening the Kundalini having at the base of our spine or the Muladhara Chakra. Kundalini came from the word kunda --"the pot" but the Kundalini Sadhna is defined as the coiled energy that looks like a serpent with three and a half coils lying dormant at the base of the spine with the tip of its tail to its mouth. The three coils represent the Three Gunas: the Sattva, Rajas and Tamas, the half coil represents the Vikritis. Anatomically, Kundalini is located in the perineum region of the body, which is between the rectum and the testicles in males and between the vagina and clitoris in females. Kundalini is also the symbol of uniformity of the masculine power called Siva and the feminine power called Sakti. When these two different sources of energy unite, they become complete and become a fundamental source of energy and power. This union happens in the brain or Sahasrara Chakra and gives us divine prosperity, universal energy, creativity, feeling of love and affection and eliminates negative feelings--depression and jealousy.

History of yoga

The history of origin of yoga is quite mysterious and shrouded. Still it is believed that about 4500 B.C. before initiating human civilization. The evidences found out so far tell that Yoga grew out during Stone Age Shamanism. According to archeological proof because of the cultural similarities between Modern Hinduism and Mehrgarh's, a neolithic settlement (in what is now Afghanistan) excavations it is prevailed the root of yoga is enrooted in this shamanistic culture of Mehrgahr. Early Yoga and archaic shamanism had much in common as both sought to transcend the human condition. The primary goal of shamanism was to heal members of the community and act as religious mediators. Archaic Yoga was also community oriented, ait attempted to discern the cosmic order through inner vision, then to apply that order to daily living. The first archaeological evidence of Yoga's existence is found in stone seals excavated from the Indus valley. The stone seals depict figures performing Yoga postures.

Yoga and you
Yoga can help Women cope with health issues like hormonal changes associated with Menopause, Pregnancy and Menstruation can cause Women to suffer from Premenstrual Syndrome, Menstrual Cramps, headaches, and other symptoms and help them develop the state of their body and mind. It alleviates the pain that goes with Menstruation, manage stress, and ensure an easier delivery among other benefits. Yoga poses are also designed to tone and exercise the muscles of the body to eliminate excess fat, and make it more flexible and stronger. 

Men are expected to be physically and mentally strong and fit to cope with stress and to meet the demands of everyday life, together with sports activities. To achieve a fitter body, they should practice Yoga to achieve a healthier body and mind to meet life's increasing demands. Not only do they need to adapt stress of being the head of the family or what others consider as the "stronger sex", they also have to deal with the demand to look good.

Growing Old is the stage when you are more susceptible to some ailments like Arthritis, rheumatism, incontinence and High Blood Pressure. This raises the need for the seniors to stay fit and healthy during this age.

So yoga, today becomes an essential part of your daily life.