Shakespeare's earliest published works are classified either as folios or quartos, depending upon the folding of the sheets. Folios are large and tall, and quartos are small and square. While selected plays were printed in quarto format at different stages in Shakespeare's career, the First Folio - the earliest edition of the collected plays - was printed in 1623 by Heminge and Condell, and contained 36 plays (many published for the first time). The text was dedicated to both the Earl of Pembroke (Wiliam Hebert) and the Earl of Montgomery (Philip Herbert). Three additional folios appeared after the first, in 1632, 1663, and 1685. These later folios were copied from the first edition, and contain omissions and mistakes.

The quartos are classified further as "good" or "bad" quartos. The good quartos were reproduced from reliable sources, such as the 'foul papers', Shakespeare's original manuscripts. Most critics now agree that over half of the quartos printed during Shakespeare's lifetime are bad, corrupted by actors trying to reproduce Shakespeare's words from memory. To read more about the folios, quartos, and the Elizabethan attitude toward publishing, please read my article Shakespeare in Print.

How do modern versions of Shakespeare's plays differ from the quartos and folios?

Shakespeare's plays have changed slightly over the centuries, and the editors of modern versions have corrected typographical errors and modernized spelling. Stage directions have also been added to the plays, and editors have divided the plays into acts and scenes. The settings of each play have also been added. The first editions of Shakespeare's plays had no references to setting at all, with the exception of two: Measure for Measure, set in Vienna, and The Tempest, set on an uninhabited island. All the other settings that we now have for the plays were included long after the First Folio, beginning with Nicholas Rowe's edition in 1709.