A denomination is an identifiable religious body under a common name, structure, and/or doctrine. Expressions of Christianity, in modern times, exist under diverse names. The greatest divisions in Christianity today however are between Eastern Orthodoxy, Roman Catholicism, and the various forms of Protestantism which came out of the Protestant Reformation.
The different forms of Protestantism developed historically along geographical and political lines, as well as by doctrinal differences, exhibiting various degrees of commonality. Groups such as Lutherans, Anglicans, Presbyterians, Methodists, etc. find their roots in the Reformation and are are commonly referred to as Christian denominations with differences in ecclesiology and theology. In some groups, congregations are part of a particular church denominational hierarchy, while in other denominations, notably Baptists and Congregationalists, each congregation is an independent autonomous local body. Local-congregation-autonomy within Protestant Christianity has grown due to the 20th century independent Bible Church movement, especially in the U.S. Technically, Bible Churches do not comprise a Protestant denomination, although they have become a significant identifiable segment.
Denominationalism may also be viewed ideologically, regarding some or all Christian groups as being, to some extent, versions of the same thing -- regardless of their distinguishing labels. Not all denominations would agree with this, however; and there are some groups which practically all others would view as apostate or heretical -- that is, not legitimate versions of Christianity.