Understanding particle points with shaders

Good morning,
Hopefully I have the right forum area. I need some help learning to write opengl/cg shaders.

I’m learning about using shaders with Ogre3d’s implementation of particles and had some questions about how the point type particles work.

Q. I believe point type particles are implemented as a single vertex. Is one single vertex is passed to the vertex shader?

Q. If one vertex is passed to the vertex shader then what is sent to the fragment shader?
From reading the cg tutorials I believe the PSIZE semantic is used to define what the frag shader
renders. I assume this value can be passed into the vertex shader, or calculated within it, and if
desired then passed to the frag shader?

Q. The tutorial mentions that the frag shader is called to render a square with the uv mapping
set. Is the position from the vertex shader at the origin of the rendered square?

I’m implementing GPU smoke and special effects.