WBDG

"Form refers to the shape or configuration of a building. Form and its opposite, space, constitute primary elements of architecture. The reciprocal relationship is essential, given the intention of architecture to provide internal sheltered space for human occupation. Both form and space are given shape and scale in the design process. In addition, the placement of a building form in relation to its immediate site and neighboring buildings is another crucial aspect of this form/space relationship. Just as internal space is created by voids in building form, exterior space can be defined or poorly defined by the building form as well.
For instance, consider the difference between an infill building that fits tightly within its' site boundaries (leaving no unoccupied space on the site, except perhaps a defined outdoor courtyard) and a freestanding building located within a large expanse of parking. Without the aid of other space-defining forms such as trees, fences, level changes, and so forth, it is very difficult for a large space to be defined or satisfactorily articulated by most singular forms.
A number of aspects must be considered in order to analyze or design an architectural form, including shape, mass / size, scale, proportion, rhythm, articulation, texture, color, and light." to find out more...
Passage from Phoebe Crisman, Assistant ProfessorUniversity of Virginia School of Architecture