Deep breathing, or breathing exercizes are one of the most important parts of a yoga practice. But anyone can benefit from breathing exercies. To properly execute a yogic breath, one must draw out the breath. If being done along with a yoga posture or asana, you should try to match the time it takes for you to move into the posture with the time it takes to inhale or exhale. For example, if you are raising your arms above your head while you inhale, you should move your arms slowly and breath in the whole time you are moving your arms.
A very simple breathing exercize that can be done anywhere (I do them at my desk at work all the time) is to inhale into your belly. Your belly should rise when you inhale, not your chest. Hold the breath for a beat, and then exhale, making the exhale slightly longer than the inhale. Try to exhale all the air out of your lungs. Do this several times in a row.
When using breath with yoga, one thing is important to remember. Inhaling is considered an opening activity, so inhale with opening movements, like lifting your arms above your head or coming out of a forward bend. Conversely, Exhaling is considered a closing activity, so exhale with closing movements, such as bringing your arms down by your side, or going into a forward bend.
In yoga breathing is considered pranayama.