Here is my take about the difference between Mathematica and the Wolfram Language: Mathematica is the combination of an interactive front end for writing code/functions etc., sending them to the Mathematica Kernel, and the displaying the results in the form specified by a functional wrapper you specified. The Wolfram Language is what you use to talk to the Mathematica kernel.

But you can also write and save WL code and submit it to the kernel directly. It will do whatever it's supposed to do: acquire and process data, save the results, and it includes all of the familiar Mathematica capabilities: image processing, integrating functions, finding the solution for an equation. You can do this in the kernel now.

The current development seems to be an extension of the range of the current WL as used in Mathematica (the front end/kernel package) into a general purpose language that includes all of the WL's current capabilities plus various new ones that were missing. The WL preliminary documentation site talks about the new stuff, but it's hard to know what's new if you aren't familiar with the current version.