"the cessation of fluctuations of consciousness" - Patanjali
"a timeless pragmatic science" - B.K.S. Iyengar
"the most versatile spiritual tradition of the world" - Georg Feuerstein
And these statements are by just three of the experts on the subject. Yoga is in fact probably all of the above. The word 'yoga' in Sanskrit means to join, to unite, to yoke - essentially meaning bringing unity, harmony, integrity.
There are many methods, styles, techniques, many schools and many teachers - the field of yoga has great diversity. Perhaps it is best to compare it to the arts - in the field of arts there are many disciplines - painters, writers, sculptors, musicians; within each of these disciplines there are different styles, within styles different artists produce their work, bringing their own interpretations, adding their own 'colour' to the vast palette. So too in yoga - there are many styles and sub-styles - for example the broader style of contemporary hatha yoga contains dynamic yoga forms such as ashtanga yoga, vinyasa yoga, power yoga; the ashtanga vinyasa yoga practice style sprouted many further sub-styles such as power yoga, vinyasa flow, sun power yoga, etc.
There are styles that do very little physical practice, and there are some that are very much driven by the physical. There is yoga for fitness, yoga for relaxation, yoga therapy - there is some form of yoga to suit most people's needs, abilities and aspirations.
It should also be noted that contemporary postural practice (Asana) is broadly misunderstood and misrepresented. Evidence continues to emerge that the posture practice we have today has been the result of the physical culture revolution around the turn of the 19th and 20th century. Sri T. Krishnamacarya, recognised as the 'grandfather' of modern yoga styles, has incorporated many influences into the systems of practice he developed, many of them of western gymnastic origins. Anyone wanting to explore this in more detail should read the book 'Yoga Body' by Mark Singleton.
And of course yoga is delivered/taught/instructed in many different ways - there are yoga classes, yoga workshops, yoga retreats, yoga one to one and yoga private tuition.
Volumes have been written on this subject - the truth is that 'yoga' is just a word. This word can be filled with many 'meanings'. What will make the difference is what it means to you. You can hear about yoga, you can read about yoga, but finding out what it can do for you can only come about from exposure to practicing yoga - as K. Pattabhi Jois said - "practice, and all is coming".