As a practice, yoga is both mental and physical, while meditation is a purely mental technique that can, but doesn't necessarily have to, incorporate physical elements. While meditation can be considered part of a yoga practice, and people can practice yoga and meditation together, the two are generally considered separate ideas.
Some people practice yoga as a purely physical activity as a way of exercising, stretching and challenging themselves. These people may or may not also incorporate meditative practice, such as awareness of breath, into their yoga. Awareness of breath is a common element in yoga practice, but an individual can also do yoga poses without bringing attention to their breathing. People who meditate may prefer to do so in a specific physical position or pose, but ultimately the focus of meditative activity is on the mental rather than the physical.