This is not meant to be an all inclusive primer of what each package is used for by any means as each can often overlap into other areas. It really depends on what you want to do, you can break it down into several groups though and some packages are found in more than one.

All around 3D package – You can model, animate and render in these. Usually the modeling is polygonal only but some packages do NURBS as well.
Maya, 3dsMax, Lightwave, Softimage XSI, Cinema4D, Carrara, Blender,
Truespace, Shade, Realsoft3D…there are more as well.

Modeling Package- These usually just provide a faster modeling environment than the all around packages above. Their interface is all about fast modeling and they can often be found working along side the all-arounders providing the meshes for them.
Modo (It can render now as well and they are working on animations it will probably change to an all-arounder in another year), Silo, Hexagon, Amapi,
Wings 3d.

Character Animation – If you set your sights on creating a wombat dancing the cha-cha in a hurry these are the apps for you. The all-arounders can do CA as well, some as good as specialist applications, some not as good.
Project Messiah, Hash Animation Master – No or little modeling but good CA and rendering
Maya, 3dsMax, Softimage XSI, Cinema4D – As good as above
Carrara, Blender, Lightwave, truespace, Shade, Realsoft3D – Not as good.

Content Apps – These focus on content bought from somewhere else and assembled/arranged for your creation. Most of the time you don’t have to model or texture the content as it was already done by the creator. Some do limited animation. It must also be mentioned that any all-arounder can use content as well.
Poser, Daz Studio, Vue.

Landscape Packages – They are really the best or fastest at producing terrains and landscapes and some can do animation, but it is limited compared to the all-arounders.
Vue (all versions), Bryce, Carrara, Terragen, Worldbuilder

Product Design – These are usually industrial strength modelers using NURBS or solids. Just about anything you lay your eyes on was probably built or designed in one of these. Some have the ability to render internal or as a plug-in.
Solidworks, Solid Thinking, Pro/E, Rhino, Alibre Design, Amapi, AutoCad, TurboCad, Alias Studio, MoI

Organic Modeler – A package with a workflow to imitate natural sculpting. If you’ve seen an incredible detailed (the pores have pores!) image in the last 3-4 years it was probably polished in one of these. The output usually needs to be sent off for rendering in another package.
Z-Brush, Hexagon, Silo 2, Mudbox.

Render Only – A plug-in or standalone app that can render like nobodies business. Like a model only package the focus makes them extremely good at what they do, give you the best possible image of your creation.
Renderman, V-Ray, Maxwell, Mental Ray, Final Render, Brazil