The vast majority of those that wear a Gartel during prayer are Hasidic Orthodox Jews; a smaller number of non-Hasidic Haredim and an even smaller number of non-Haredi Orthodox Jews (or even non-Orthodox Jews) may also wear a gartel during prayer.
Gartels are generally very modest in appearance. Most are black, but some gartels are white, possibly to be used on some special occasions such as Yom Kippur, and cost more than black counterparts of the same width. Gartels are composed of multiple strings, anywhere from four to over forty in number.
Jewish law (Shulchan Aruch O.C. 91:2) requires that there is a physical divide between the heart and the genitalia during prayer. It is commonly explained that separating the upper and lower parts of the body manifests a control of the animal instincts of the person by the distinctly human intellect. Hasidic authorities maintain that a regular belt or the waistband of trousers do not suffice to fulfill this requirement, and that a designated sash is to be used. Additionally, donning a gartel is a preparation for prayer, in accordance with the line "Prepare to meet your G-d, O Israel" in the Biblical verse, Amos 4:12. In contrast, Mishnah Berurah (91:5) states that any waistband is adequate for the first requirement, however for the second requirement more is needed, which the gartel fulfills.
Some Hasidic groups such as Skver and Belz wear the gartel all day as part of their regular attire. ja:ガルテル