The rasterization stage converts vector information (composed of shapes or primitives) into a raster image (composed of pixels) for the purpose of displaying real-time 3D graphics. During rasterization, each primitive is converted into pixels, while interpolating per-vertex values across each primitive. Rasterization includes clipping vertices to the view frustum, performing a divide by z to provide perspective, mapping primitives to a 2D viewport, and determining how to invoke the pixel shader. While using a pixel shader is optional, the rasterizer stage always performs clipping, a perspective divide to transform the points into homogeneous space, and maps the vertices to the viewport.
Vertices (x,y,z,w), coming into the rasterizer stage are assumed to be in homogeneous clip-space. In this coordinate space the X axis points right, Y points up and Z points away from camera.
You may disable rasterization by telling the pipeline there is no pixel shader (set the pixel shader stage to NULL with ID3D11DeviceContext::PSSetShader), and disabling depth and stencil testing (set DepthEnable and StencilEnable to FALSE in D3D11_DEPTH_STENCIL_DESC). While disabled, rasterization-related pipeline counters will not update. There is also a complete description of the rasterization rules.
On hardware that implements hierarchical Z-buffer optimizations, you may enable preloading the z-buffer by setting the pixel shader stage to NULL while enabling depth and stencil testing.