When people speak of Tai Chi, usually they speak of it as a single style, even (in my experience) among those who are knowledgeable about martial arts. At first look, they do look very similar in that they each catalog and teach its techniques through the use of extensive forms practice at slow paces. They seem to have very similar techniques. So what makes one style of Tai Chi distinct from another?
The above is a summary of information found in a compact yet readable manner on this website, which I really recommend as a read on TaiChi, as it has a lot of very good information on Bagua, Xing-Yi, and some fairly fundamental concepts including how to stand / good posture and a page on the philosophy of martial arts.