Basically, there are 2 types of 3D software, solid modelers and surface modelers. Both have their strengths and weaknesses. And while there are no hard and fast rules, here are some generalities.

Solid modelers
More expensive
Primarily uses booleans for constructions - add, take away like a machining process
Great for assemblies
Great for simple shapes
Industry standards are ProE and Solidworks

Surface modelers
Cheap
Uses NURBS for constructions - non-uniform rational basis spline, a lot of pushing and pulling
Horrible for assemblies
Best for complex surfaces seen in automotive and many consumer products
Industry standards are Alias and Rhino

Both have similar learning curves. Your local community college will have classes in solid modeling and not surface modeling.