I agree with Al - take a look at that link to its home page.
I would describe Visio as a technical drawing package. It is less powerful,
and far easier to learn, than most CAD packages, but remarkably capable and
actually far more suited to most people's needs.
It is resolutely 2D: unlike modern CAD programs there is no extrusion,
lathing or rotation in the Z axis.
I regard it as one of the finest pieces of software available today. Of
course, it isn't really a Microsoft product - they bought it from Visio Corp
several years ago.
If you ever need to produce diagrams more sophisticated than the drawing
tools in Word, PowerPoint, etc can manage, then Visio is the one to use.
As a technical author I couldn't possibly work without it.
Steve