Time window

Time windows were doorways in time that allowed time travel.

Functions
Time windows could be created by pushing energy into a warp drive, as was done on the SS Madame de Pompadour. (TV: The Girl in the Fireplace)

One type resembled a portal linking two time periods, which had the appearance of shattered glass. The only person known to control these time windows was the mysterious Shopkeeper, who could send other people through them by clicking his fingers several times and pushing the windows towards them. Unlike the time windows created by the SS Madame de Pompadour, these versions could prove dangerous if left open, eventually causing the entire Earth to be sucked into the Time Vortex. (TV: Lost in Time)

The Doctor's TARDIS was also capable of opening time windows. (TV: Death Is the Only Answer)

History
In 2010, the Shopkeeper sent Sarah Jane Smith, Clyde Langer, and Rani Chandra to 1889, 1941, and 1553 respectively to find three objects to save time. The amount of time they could spend in the time zones was dictated by a sand timer which, if it ran out, would leave them trapped. (TV: Lost in Time)

The Clockwork Droids created time windows placed throughout the life of Jeanne-Antoinette Poisson onboard the SS Madame de Pompadour. They were closed when the Tenth Doctor broke the link by shattering one window. A single one remained open due to a loose connection, allowing him to travel back to when he had come from. He then closed it with the TARDIS. (TV: The Girl in the Fireplace)

The Eleventh Doctor accidentally opened a time window inside his TARDIS by dropping his fez on the controls, summoning the fez's original owner, Albert Einstein. (TV: Death Is the Only Answer)

He also purpose created one and hid it inside a box as a Christmas present for Madge Arwell and her children, Lily and Cyril, to tour of one of the most peaceful planets he knew. (TV: The Doctor, the Widow and the Wardrobe)

Behind the scenes

 * The time windows used by the Shopkeeper resembled the anomalies from the ITV sci-fi series Primeval.