The word yoga means "union" in Sanskrit, the language of ancient India where yoga originated. We can think of the union occurring between the mind, body and spirit.
What is commonly referred to as "yoga" can be more accurately described by the Sanskrit word asana, which refers to the practice of physical postures or poses.
Asana is only one of the eight "limbs" of yoga, the majority of which are more concerned with mental and spiritual well being than physical activity. In the West, however, the words asana and yoga are often used interchangeably.
Many people think that yoga is just stretching. But while stretching is certainly involved, yoga is really about creating balance in the body through developing both strength and flexibility. This is done through the performance of poses or postures, each of which has specific physical benefits. The poses can be done quickly in succession, creating heat in the body through movement (vinyasa-style yoga) or more slowly to increase stamina and perfect the alignment of the pose. The poses are a constant, but the approach to them varies depending on the tradition in which the teacher has trained.
Yoga teachers will often refer to "your practice," which means your individual experience with yoga as it develops over time. The amazing thing about yoga is that your practice is always evolving and changing, so it never gets boring. Although the poses themselves do not change, your relationship to them will. Anyone can start a yoga practice, even if you don't feel like you are very flexible or very strong. These things will develop over time. Another great thing about thinking about "your practice" is that it encourages the noncompetitive spirit of yoga. One of the most difficult, but ultimately most liberating things about yoga is letting go of the ego and accepting that no one is better than anyone else. Everyone is just doing their best on any given day.