[mthd]

to add some more specifics (i design these things...)

all tall buildings require elevators, stairs, and vertical distribution of water, air, electricity, data, etc.

they also all require lateral force resisting structural elements, for wind and (in some regions) earthquakes. these elements can be shear walls, typically arranged in rectangular formations near the center of mass of the building, or frames of various types including steel moment frames (columns and beams joined together to form rigid connections at their corners), concrete moment frames, or diagonally braced frames. the latter is not usually seen with concrete because concrete behaves badly in tension, and any diagonalized frame will see tension as often as compression.

all of these elements - elevators, stairs, shafts, and walls, frames, or braces, can be arranged according to the architect's and engineer's desires. however, it tends to be more efficient to group at least some of them together, and then surround those things with structural elements. this is the "core" as we typically refer to it in a modern high rise building.

on certain kinds of sites, or for certain kinds of buildings, these things are grouped together but not put at the center, allowing more open floors. this can be difficult due to modern restrictions on the placement of the exit stairs. it was more common in the 50s and 60s.

one important addition starts to occur as buildings get either very tall or very skinny: the efficiency of the lateral resisting elements starts to get very poor if they are themselves very tall and skinny (beyond an aspect ratio of 15 to 1 or so, so a 40 foot "wide" core for a 600 foot tall building, etc.) and this leads to the desire of both engineers and architects to put these elements on the *outside* of the building, and sometimes make them diagonal. these buildings usually still have cores, just with reduced lateral resisting elements...