Architecture and Building Construction are not the same thing. Architecture focuses more on the art aspect of a building, while building construction focuses on how to physically create such a building. Building construction is important, you must get the scale and proportions right in order to create comfortable space for human activities. Of course, architecture is important for the building design as well, adding qualities such as color, patterns, texture, etc. One cannot work without the other. Structural systems are basically what supports the building, which would be columns, beams, etc. Enclosure systems is the “shell” of a building. A roof would be considered a “shell”, same with walls. Mechanical systems is the “essential services to a building”, water systems, sewage disposal.