How does one make products for Poser? Do you have to have Poser to make clothes or another person to be used in the program? Or can you use any 3d program, like Blender 3d, and make clothes or people that can be used in Poser?

Any modeling program can be used, Shade, Wings, Rhino, etc. Just save it in .obj format as that is Poser's prefered format.

The actual creation (mesh-making) is done with a 3D modelling application like Blender. UV-mapping and texturing are also done with third-party apps.

Rigging a figure (to make it posable) generally requires tools included in Poser. Even if you use an approach other than Poser's Setup Room, you'll likely still need the Joint Editor and Hierarchy Editor at some point.

Morphs can be created either with Poser's magnets, or a modelling application.

Either way, you would definately need Poser to test it properly:)

So what would I need to buy? Is Poser Setup Room in the latest Poser, or is a seperate program? Where would I get Joint Editor and Hierachy Editor? Currently I've made a promise to myself to buy Maya, would any applications in this program be useful, or am I better off going with another program?

All recent versions of Poser (P4, P5, P6) have the Joint Editor and Hierarchy Editor. The Setup Room was introduced in the Pro Pack add-on for P4, and has been included in all versions since.