A Hatha class basically consists of two elements of yoga-Prana and Asana.
Asana translates to posture or pose, so in a class when you hear the word bakasana it means crane pose; baka-crane, asana-pose. All Asana, no matter how basic or challenging, are simply designed as an experience for you body to breath in. The point is not to achieve the Asana but to explore it with your breath, while maintaining focus and attention to the body and to the breath. Therefore the depth of any pose is not measured in how far or deep you physically go, but rather your ability to breath freely and fully no matter where you end up.
Prana in yoga is referred to as the breath, and translates to life force (again, think Star Wars). Pranayama is the extension of life force (aka practicing breath). In a Hatha yoga class we practice the style of breath known as Ujjayi, which translates to victorious breath. This is a diaphragmatic breath, done with the lips closed breathing through the nose. Ujjayi breathing is commonly referred to as 'ocean breath' since when practiced correctly, makes an ocean wave like sound (it has also been referred to as Dark Vader breath, however not as harsh.) This breath is not to be practiced forcefully but rather with ease adding a light constriction in the back of the throat (glottis), to create and even rhythm. The breath warms the body from the inside, and cools the mind-as its sensation and sound are a focal point to draw your attention towards throughout a yoga practice.