As a genre itself, trance can be hard to define. The main characteristic of trance that separates it from other kinds of dance music is the emphasis of melody, 4/4 beats and a general "uplifting" sound. Within the genre, there are subgenres, and while the list of trance subgenres is seemingly ever-growing, some are more popular than others.
Acid trance came from acid house and is one of the earliest forms of trance, combining the hard, abrasive sound of that acid house with the hypnotic pulsing beats of early trance. While most trance is often described as uplifting and melodic, acid trance is little of either. Instead, it's more beat-driven and less emphasis is placed on the vocals, creating a darker and heavier sound.
Progressive trance is the opposite of acid, which is a little surprising, considering it's an offshoot of that genre. Everything is smoother and more melodic, the rhythm is further back in the mix and the beat is less constant and pounding. Of all the trance subgenres, progressive is one of the most pop-friendly. While it is still created for the dance floor, many progressive trance artists are more focused on creating solid albums of music than dance singles.
Also called "anthem trance" (although some would say the two are different genres), uplifting trance is just that–inspiring. While other subgenres of trance build their songs slowly to create a climactic peak at the end designed to get everyone moving, uplifting trance is almost nothing but peaks.
Vocal trance is kind of self-explanatory. While other trance subgenres may have vocals, the emphasis here is solely on the singing. Vocal trance almost has the same structure as pop music, and even some pop songs have been covered by vocal trance artists to wide mainstream appeal.
Goa trance takes its name from a region of India known for its intense drug-heavy club scene. It has a strong psychedelic bent, no doubt because of the heavy usage of LSD in the scene from which it came. Because of its region of origin, it's not uncommon to hear Indian instruments such as the sitar mixed in with the electronic sounds and samples.
Related to Goa but still a genre of its own, psychedelic trance is less melodic than goa and features a harder sound reminiscent of acid trance. However, the beats in psychedelic trance tend to be faster than in acid, and the sound can be described as more "sci-fi," with space-themed tracks not uncommon.