In the days when the Talmud was compiled (and certainly in the times of Moses), there were no high rise apartment buildings. Families lived on farms or in small villages, and those who lived in cities, lived in dwellings that were rarely more than a few stories high. With modern architecture and the invention of the electric elevator at the end of the 19th century, urban Jews who were shomer Shabbat (Sabbath observant) were faced with a new halachic (Jewish legal) dilemma. May one use an elevator on Shabbat?
The question of the elevator seems simple: Pressing a button to either call an elevator or designate the desired floor necessitates the use of electricity, which was deemed to be an aspect of several m'lach'ot (prohibited creative labors), including of Boneh (building) and Makeh B’patish (the final hammer blow). However, if one did not press the button, but merely followed a non-Jew into an elevator and took it to the same floor as the other passenger (and then took the stairs the rest of the way), one would not seemingly have any contact with the usage of electricity.
Those aware of the technology, however, became concerned that the additional weight of even one person increased the electrical currents and affected the operation of the elevator. Where technology appeared to be in conflict with halacha, great minds persevered to find ways to alter the technology to comply with halacha. From such a process was born the "Shabbat elevator."
A Shabbat elevator is preset to stop and open on every floor (or every other), has had its non-emergency manual controls disabled, and its weighting mechanisms neutralized. Appropriate safety concerns, such as a warning buzzer before the door closes, are also part of the system.
*There are many opinions regarding the use of Shabbat elevators. If possible one should ask one's rabbi before using one.