Sidebars are little sections of articles that appear alongside the normal text. They should be used sparingly, as they don't, as of 2012, display on mobile devices. However, there are some characters, like the Master, whose histories are so complicated and contradictory that sidebars are almost essential to explaining the history of the character.

Essentially, sidebars give you the opportunity to pull the reader out of the "in-universe perspective of the article to briefly explain the publishing/broadcast situation that led to the narrative confusion.