Poser works from loading characters, and morphs, but no real modelling abilities.
that said, you can buy new figures to use, so no you aren't limited ones included in poser. Most popular figures are sold by www.daz3d.com or so it seems, which include hundreds of morphs, giving a very large amount of control of the look, while ones from E-frontier tend to have less, but in both cases you can buy more morph packs, or make your own. it takes some work to create your own morphs (from scratch) if you decide to avoid the included ones. character packs (sold in marketplace here and other sites) for the figures are generally just saved sets of the morphs already included, some very good and well worth it.

you can't really model clothing in poser, but once made you can pose it, and even turn into dynamic clothing which looks more realistic, same for dynamic hair. 

in all, it's good for posing for a scene, and fully capable of rendering many things. Material room is very good, though lacking some higher end tools (using math nodes to place step by step actions instead of a text window or something to type formulas in to manipulate the shader). 

i still like poser as my main app for posing and dynamics, but that's about the end, as i switched to carrara5 for rendering, and lighting/shaders is never a great idea to export over, decent, but not up to what i try for in quality.

edit: if you intend to use poser for games, majority of figures included and sold are WAY too high poly to use, there may be a few low enough, but haven't seen