Yes, you can use morphs to change the look of a figure. The extent to which this can be done depends on the figure. Most of the figures that come with poser have the morphs "built in", ie when you load the figure you can adjust the morphs in the parameters window. Some figures such as Dawn require you "inject" the morphs, this is a pose file you apply to the figure. You can purchase extra morph packages for some figures such as LaFemme and Dawn/Dusk.

I'd suggest you just have a play with poser, then post a thread in one of the other poser boards (new users etc) when you have specific questions.