Every calendar in the world solves the same arithmetic problem, and there are only three solutions. A lunation is about 29.53 days and a tropical year about 365.2422 days, and the second is not a whole multiple of the first. A calendar can therefore follow the Moon and let its year drift against the sun, follow the sun and let its months lose contact with the Moon, or follow both and insert an extra month at intervals to keep them together.
The Hijri calendar takes the first path and takes it without compromise. Twelve lunar months, no intercalation of any kind, a year of 354 or 355 days, and consequently a steady drift of about eleven days a year against the seasons, so that Ramadan moves through the whole solar year in roughly thirty-three years. The Quran is explicit that the months are twelve and that intercalation is not to be practised, and the prohibition is understood to have ended a pre-Islamic Arabian practice of adding a month.
That makes it the only major calendar still in wide use that is purely lunar, and the drift is a feature rather than a defect: the fast and the pilgrimage pass through every season, so no part of the year is permanently easier or harder for any latitude.
A second distinguishing feature is that the month traditionally begins on the sighting of the new crescent, which is an observation and can differ between places and between authorities. Calculated versions exist and are used for civil purposes and for printed calendars, so a Hijri date may be given differently by two sources on the same day, and the difference is a matter of method rather than of error.