What exactly is the difference?

The difference is that xstream is a plugin to be used in conjunction with other high profile programs like C4d,XSI,Maya,3DS.  In other words, you can work in those programs and render VUE elements into them.  So, open VUE, create a terrain, add an ecosystem, populate it and you can enjoy that element in the other program.  Check e-on's site, they list all the advantages of xstream and a comparison between xstr and inf.

 Perhaps "in conjunction with" understates it a bit. XStream is a version of Vue to be used INSIDE other modelling programs (specifically C4D, Maya, Max, Lightwave, and XSI).

Because it is intended to be used this way, it is a more complicated program, and costs more.

Ther primary reason a person might want to be able to do this is because they are making animations or using the "dynamics" or particle processes these modelling programs provide. So, the Vue  'environment" is provifding an environment for these animations, etc. But, of course to do this, you must have one of these programs, a fairly powerful hardware set, and you are making the renders within  one of these programs. Even then, because of the extraordinary demands being made by having all this going on, there are compromises on how well the Vue plugin is able to work inside these programs.

Long story short, if you do not intend or need to build Vue scenes inside one of the above programs for these reasons, you want to purchase Vue Infinite, rather than Vue Xstream!!!!!