I've learned a lot since my first post.  The main difference between Daz Studio and Poser is that DS is a stage to modify, dress, and pose the figures.  Poser is a scene and creation software.  There is a software bridge between the two programs that allow you to move things between the two programs.  You create in poser then move to DS for morphs, clothes and posing (you can also do scenes) and the transfer them back to Poser for final render.

You also can do it all in DS but you have to start with their base figures.  Other people make morphs or characters that can be used in DS to create complete renders.

DS is compatible with most of the things on sites like ShareCG.  It's easiest to open the runtime zip and copy each part into the DS runtime (geometries, props whatever)  Some file types you can't use like pmd for poser, but for the most part they are pretty compatible.

There is now a DazStudio 4, that has many more features and can do things it never could before.  Right now it is free, but will cost money in the near future.  It's worth getting just to have it and/or play around to experiment.