Highly observant and intensely religious-that would be the best sociological description of strictly Orthodox Jews, otherwise known as Haredi. For a demographer, the reason to talk about Haredi is their demographic uniqueness.
Their fertility today, mostly 6-7 children per woman, is the highest in the world. It is generally not nearly as well known as it should be but today populations of the Middle East and sub-Saharan Africa have less children than Haredi.
Haredi longevity is high. It is as high, if not higher, as longevity of non-Haredi Jews. The link between wealth and longevity, widely observed across societies and well-understood by epidemiologists, simply does not apply in Haredi case.
High fertility combined with high longevity generates an extremely high rate of population growth: around 3%-4% per annum. At this rate, the number of people in a population doubles approximately every 20-25 years. If this rate persists, the population of, say, 76,000 British Haredi around 2020 could grow to 152,000 in 2040 and to 304,000 in 2060. Given that the non-Haredi component of Jewish populations in the Diaspora is numerically stagnating in many locations, appearance of Haredi majorities in some Jewish populations in the not-too-distant future is not beyond the realm of possibilities.