Historians love to create historical periods or eras and then use them to make sweeping generalizations about the culture and worldviews of vast nations across whole centuries of time. Conversely, an only slightly less popular pastime among historians is to exhaustively point out the flaws in accepted periodizations and cataloguing in detail the historical trends that crossed period boundaries. Indeed, there seems to be a whole species of historical monograph dedicated solely to proposing new periodization schemes. But despite its weaknesses, periodization can be a useful tool in understanding the past, and in any case is here to stay, so you might as well get used to it and figure out what those crazy terms mean.
It almost goes without saying that different times, places, peoples, and cultures would require different periodization schemes and indeed, this idea seems to be borne out by the fact that every country seems to have its own native periodization scheme. In China the past was measured according to dynasties such as Han or Qing. Japan traditionally measures years according to emperors' reigns, starting over at '1' whenever an emperor dies. France measures its recent history in terms of "republics" (currently in the Fifth Republic), while Hitler, at least, viewed German history in terms of reichs.
Nevertheless, 20th century western historiography has arrived at a grand and sweeping single historical periodization scheme that has been applied not only to western cultures, but to all cultures. Essentially, historians decided to call the period they were presently living in "Modernity" or the "Modern Period" and then worked backward from there, arriving at the "Early Modern Period" and before that, the "Premodern Period," which is now often broken down further into the Medieval, Late Classical and Classical Periods. Initially, this periodization was applied only to the case of European cultures, using European historical events as the transition points and mileposts, as follows:
The Premodern Period extends from the dawn of time until 1492, when Columbus "discovered" the New World, initiating the so-called "Age of Discovery" and the Spanish expelled the Moors from Spain and initiated the Spanish Inquisition. These events also coincided roughly with the beginning of the "Renaissance" burgeoning of art and scientific enquiry. Within the Premodern Period, the Classical Period extends from the rise of the Greek city states ca. 400 BC to the onset of the Barbarian Invasions and the collapse of Roman authority circa AD 180, the Late Classical Period covers the period from 180 to circa 500 when Roman culture still predominated in Europe, and the Medieval Period covers the era from 500 to 1492 when feudalism arose and comprised Europe's predominant political system.
The Early Modern Period, featuring the Renaissance, the Age of Discovery, and the Scientific Revolution, extends from 1492 until 1648, when the Peace of Westphalia ended the Wars of Religion and brought on the rise of the nation-state as the dominant international actor on the world stage.
The Modern Period which extends from 1648 until at least the mid-20th century, and is marked by the development of the nation-state, and beliefs in the inevitability of human progress and the possibility of objective truth.
In the 70s, French philosopher Jean-Francois Lyotard added "postmodernism" to this periodization, declaring western culture to have entered a Postmodern Period in which objective truth was denied in favor of subjective reality and cultural relativism.
Even with the addition of postmodernism, this periodization scheme is essentially neo-Marxist, in that it is predicated upon an idea of human progress - of stages of human development that presumably all cultures and societies go through, although not necessarily at an equal rate. Rather than Marx's stages of feudalism, capitalism, and ultimate communism, the process became a path of modernization from premodern to early modern to modern and finally postmodern stages of development. The scheme has become utterly ubiquitous across all historical fields, such that it is completely commonplace to speak of "premodern China," "early-modern India," or "Japan's modernization process."
Naturally, Europe modernized the fastest under this scheme, with the rest of the world gradually catching up and passing into the different periods at different dates. Japan, for example, is held to have reached early modernity in 1600, following the Battle of Sekigahara, and arrived at modernity with the Meiji Restoration of 1868. China was a bit slower, reaching the early modern period with the initiation of the Qing Dynasty in 1644 and arriving at the modern era with the overthrow of the Qing in 1911. India was even slower still, finally getting out of the premodern era with the Indian Mutiny of 1857, and only achieving modernity with independence from Britain in 1947!
I leave it as an exercise to the reader to determine the worth or validity of these labels, but either way, they are utterly ingrained in the terminology of scholarly discourse and are definitely here to stay.