How we change what others think, feel, believe and do
Period dramas take relatively standard basic stories and cast them into historical settings.
A typical period drama is set in a beautiful and large country mansion house, in which the parents worry about marrying daughters off to wealthy suitors, whilst the young people are more concerned about fun and love (so no real change from the modern setting then).
Period dramas may also include war and other excitement. This may be a backdrop to the real story or may be integral to the plot.
The stories of Jane Austen, such as Mansfield Park
Period dramas often reflect the important history of the country in which the drama is set. Thus, for example, there are many American period dramas set in the time of the US Civil War.
Period dramas are often romantic as they use the idealized past as a frame in which to portray an idealized story of innocence, dastardly deeds and robust heroes. In the same way that fantasy setting allow for idealism, so also do period dramas remove the setting (and hence the audience) from the modern day and hence suspend reality and allow for idealistic characters and storylines.
One particularly genuine type of period drama was actually written in the period in which it occurred (and was thus 'modern' at the time). It is no coincidence that these stories were typically written by educated people and hence focused on what they know or aspired to, typically the dalliances of the aristocracy. People like Charles Dickens, who wrote about common people, were relatively rare.
Romantic period dramas are sometimes called bodice rippers, possibly from the archetypal book cover which shows a dark and swarthy man grabbing a woman and ripping her bodice.
Organizational stories can use a period setting to cast stories about their history into a realistic light.