'Yoga' means 'union' - of Individual Consciousness with Universal Consciousness
There are various paths of yoga that lead toward this goal.

Hatha Yoga ~ the most mainstream Yoga in the western world.  Hatha Yoga is a system of physical postures; or asanas whose higher purpose is to purify the body, giving one awareness and control over its internal states and rendering it fit for meditation.

Yoga has endless benefits for your body, mind, and spirit.  It is an excellent tool for stress relief. Hatha Yoga blends strength, flexibility and balance which can bring a tremendous amount of focus into just about anything you do in your daily life. It tones the muscles along with the vital organs of the body thus encouraging Homeostasis, the balance within the body.  When you practice Yoga on a regular basis, you begin to find that it becomes second nature to find that calmness inside of you.

Yoga is a science that has been around for over 5,000 years. It is also a philosophy and a way of life for many.  In the beginning Yoga focused deeply on the mind and spirit.  In ancient times, to meditate to the fullest, one must assume the lotus position.  To do this, the body needed to be flexible and strong. This is what has led to the physical practice of Yoga, which is known as Hatha Yoga.  Also finding that the movements of Hatha Yoga balance the bodies energies for meditation.  By the way, it is the twenty-first century now, and if your body does not want to get into the lotus position, you can still have a successful yoga and meditation practice! 

Our Western culture has captivated Hatha Yoga as a fitness tool-
yet more and more people are realizing that there can be much more to it.

Quite Honestly, Yoga can affect you in any way you choose, with even the slightest movement or stillness. When you take it slow, and pay attention to your body, even in the most basic pose, its amazing what you might find.

Karma Yoga ~ selfless service to others as part of one's larger Self, without attachment to the results; and the performance of all actions with consciousness of God as the Doer.

Mantra Yoga ~ centering the consciousness within through japa, the repetition of certain universal root-word sounds representing a particular aspect of Spirit.

Bhakti Yoga ~ all-surrendering devotion through which one strives to see and love the divinity in every creature and in everything, thus maintaining an unceasing worship.

Jnana Yoga ~ the path of wisdom, which emphasizes the application of discriminative intelligence to achieve spiritual liberation.

Raja Yoga ~ the royal or highest path of yoga, formally systematized in the second century BC by the Indian sage Patanjali.  Raja Yoga combines the essence of all other paths.

 

some great articles and interpretations:

Paramahansa Yogananda
Patanjali's Yoga Sutras
The Eight Fold Path 
Yoga Research & Education