It takes a while to get used to Poser's structure and what goes where.

Depending on which version of Poser you have, there's different things you can do.  Poser 7 allows the most freedom and you can put folders anywhere in the library and mix props and figures in the same folder. Poser 5 and 6 allow nested folders and you can have Females > Adult > Daz. Poser 4 is stuck with everything in a single folder, but I'll assume you've got something more current.

Library content belongs in the Libraries folder. If you've got P7, you can group everything there however you want (check the manual for details because I haven't played with this feature yet).  P5 & P6 require different types of content in specific folders. Figures (.cr2 or .crz--the "z" at the end means it's compressed) must be under Character. Props (.pp2 or .ppz) just be under Prop. Within a single Library category, such as Character,  all  character files (.cr2/.crz) can be placed anywhere in Character, nested however you want.
Library content, in a Library category, calls out al lthe other files needed to load something into Poser. Those files are written to tell Poser where everything is and how it's put together. If the file ends in "2", it can be read in a text editor and even edited (don't do that if you're new--if you do always save to a new file because a mistake will prevent a file from loading and you'll have to reinstall it). Any file, that has a Library thumbnail, can be moved without worry. There are some Injection files (morphs for Victoria 3, Michael 3, and other recent figures) must not be moved, but they don't have thumbnails that can be viewed in the Library.

Files, not in the Library, should not be moved around as a rule of thumb and there's no reason to move them. As mentioned above, the Library file tells Poser where everything is. Move a Geometry file (.obj) out of place and Poser will ask where it is when you try to open it. If you don't know where it is, Poser cannot load it. Textures can be moved around, but there's no reason to do so and, although Poser will eventually find the file, it will load faster if you leave it where the installer put it. You want to organize the stuff you see in your Poser Library folders, with the thumbnails that show you what everything is. You can not view geometry files or textures in your Library listings, so there's no reason to move them.

There are other Poser add-ons that assist managing content, but I've always been satisfied with just managing the Library with nested folders. Eventually, I'll get around to learning what P7 does.

My visual indexes of Poser content are at http://www.sharecg.com/pf/rgagnon