Hatha is a general category that includes most yoga styles. It is an old system that includes the practice of asanas (yoga postures) and pranayama (breathing exercises), which help bring peace to the mind and body, preparing the body for deeper spiritual practices such as meditation.
Today, the term hatha is used in such a broad way that it is difficult to know what a particular hatha class will be like. In most cases, however, it will be relatively gentle, slow and great for beginners or students who prefer a more relaxed style where they hold poses longer. It can vary a lot, so it is a good idea to call the studio before attending the class.