Hi Isabeau,
It's a 3D modelling program mainly for creating scenes with figures (posing). Other programs are used for creating 3D objects which can be used in poser (silo, cinema4d etc.).
Poser has been around for a while now so it's quite good at what it does though other artists prefer to use a different program to render their scenes.
In the hands of some artists (Restif) the renders are getting really difficult to tell from actual photos and for others such as me it might take another generation or two for that.
The current generation of figures are very detailed but the downside is that you need quite a good system to work with them.