46% or 6.45 million of world Jews live in the USA today

There are six denominations of Judaism in the US:

• The Reform movement is America's largest group - and many of its members proudly connect to the "Reform" part, appreciating their denomination's historical emphasis on prophetic Judaism and social action, personal choice in ritual matters, and embrace of patrilineal and matrilineal descent (considering as Jewish children those whose fathers or mothers are Jewish, in contrast to traditional Jewish law, which considers only those with Jewish mothers or acceptable conversions to be Jewish). At the same time, the Reform movement has in recent years begun to embrace traditional observances it shunned a generation ago, signaling a new affinity for Jewish ritual among many Reform Jews. In 1999, the movement issued a set of guidelines known as The Pittsburgh Platform, which encourages Reform Jews to study Hebrew and Torah, observe Shabbat, and recognize the importance of mitzvot (commandments).
• The Conservative movement represents a shrinking proportion of the Jewish population, though it is also seeing rising synagogue attendance rates and increasingly strong educational institutions. The range of observance within the movement is wide, and many observers have commented on the wide gap between the observance level of Conservative clergy and laypeople.
• Reconstructionist Judaism, the smallest and newest of the major denominational groups, has seen increased growth in recent years, and has benefited from the fact that its members have made an active choice to be affiliated with the movement; because of the denomination's small size and youth, most congregational members do not attend "by default" - because they have longstanding connections to the movement or because it is the only available synagogue option - but because Reconstructionist Judaism speaks to them.
• Orthodox Judaism has attracted growing members of non-Orthodox Jews to its ranks. Orthodox communities are increasingly vibrant and well-educated, and ritual observance has become increasingly stringent and conservative. At the same time, Orthodoxy has become more withdrawn from and wary of the broader secular culture. At the same time, feminists and other liberal-minded Orthodox Jews have challenged this shift to the right; their synagogues, schools, and other institutions ensure lively diversity and debate within the Orthodox world.

These four movements are generally considered to be the major denominations, but other groups also are categorized as denominations:

• Secular Humanist Judaism believes in cultural Judaism without belief in God or traditional observance. It ordains rabbis and has temples for services.
• Jewish Renewal has infused Judaism with meditation, chanting, and other popular elements of contemporary spirituality and kabbalah (Jewish mysticism). It, too, ordains rabbis and has affiliated synagogues.

Perhaps in response to the trouble and pain that such splitting of communities can cause, other Jews are seeking "post-denominational" communities, in which issues of denominationalism are no longer relevant. Such Jews want a focus to be on K'lal Yisrael -- what unites, rather than divides, the Jewish people.

Source: http://www.myjewishlearning.com/xcommon/Hot_Topics/Denominations_Index.htm