"Hatha" is the generic term for the physical practice of yoga. Remember that yoga is a multi-disciplinary practice -- what Westerners typically think of as yoga is the physical practice, but that is just
part of yoga's meditation practice, and yoga
also includes a spiritual practice, a service practice (where we get the word "karma" from), and an intellectual practice.
Within Hatha Yoga there are a variety of styles. One of the major style-groups is vinyasa, which involves flowing movement (as opposed to holding poses, although that rather simplifies the distinction). Ashtanga is a relatively
intense vinyasa practice. Power Yoga is very similar in intensity, but doesn't have the trappings of ties to the specific yoga master that Ashtanga derives from.
When a studio offers, specifically, Hatha yoga, they're typically
not offering a vinyasa practice. Vinyasa is almost always highlighted in the description of any offering, either by application of one of the names of the practice (such as "Ashtanga") or by description.
Back to the original question: I actually did yoga (specifically the Kripalu style of Hatha yoga) for several years prior to losing my weight. So, I suppose you could say that you could do yoga and still not lose weight. However, that isn't to the yoga didn't have impact. It strengthened my body, helped manage some serious back-pain (which turned out to be a problem, but through no fault of the yoga -- it's a long story and not very relevant to the question

), and got me fit enough to do vinyasa.
Vinyasa was a very important part of my weight-loss efforts. It's a nice balance between aerobic and anerobic exercise, the former helping burn calories necessary to lose weight, and the latter helping build lean-body mass necessary to keep weight off long-term. I practiced at least twice a week, at least 90 minutes each practice. Combined with my eating program, and walking five times a week, that was enough to help me lose 100 pounds.
It is almost impossible to do vinyasa and not work hard -- one of the advantages of vinyasa IMHO. However, it's kinda hard!

I remember several times when folks at our yoga studio -- folks who were coming to our studio for the meditative benefits of yoga, the relaxation and stress-release -- they'd have missed their class and came to the vinyasa class to collect on their "make-up". Gosh, were they surprised.

It
is possible to do regular Hatha yoga without breaking a sweat. There is nothing magic about yoga, from a calorie-burning perspective: A heart-rate monitor will indicate whether it is helping you lose weight or not. By the same token, you could still be building muscle if you're holding poses long enough.