Hatha Yoga is a general term that describes yoga classes that primarily consist of yoga postures. In this sense Ashtanga Yoga is a form of Hatha Yoga. You could go to 10 Hatha Yoga classes and they could all be noticeably different depending on the teacher's interpretation.

The main differences between my own Ashtanga and Hatha classes are that the Hatha classes are physically less energetic with more of a variety of yoga postures and breathing & relaxation techniques.

With the Ashtanga system you practise in a pre-set order of yoga postures. In the Hatha classes, although there will be a general pattern to the class, it will vary slightly from week to week.

The Hatha classes will be more accessible to a wider range of people.