Wireframe is a viewport display setting that lets you view objects in a given viewport as a wire mesh. This is the default setting for non-Perspective viewports. You change this setting from the Shading viewport label menu.

Wireframe mode display of a director's chair and megaphone

In addition, you can set the Standard and Raytrace materials to render as wires. Use the Extended Parameters rollout to set the size of the wire, and specify its measurement in either pixels or units.

When you use pixels, the thickness of the wire is based on the screen pixels. Therefore, it's absolute, and remains the same, regardless of its distance from the camera. If you use units, the thickness is based on world units, and varies depending on the distance from the camera. It's easier to compare the effect of pixels and units if you first adjust the camera view to give a greater sense of distance. You can most easily do this with the Perspective viewport navigation tool, which dollies the camera in one direction while changing the field of view in the other.