@ the most elementary level:
1. Usability: the state of being "usable", applied to anything that one might "use" to perform X task, and the fact that it can be used, i.e., it "works" / is in working order. Whether the item in question is easy or difficult to use is addressed in your second question...
2. "Ease of useage" (Note: Better = "ease of use", often in American English rendered expressed as "user friendly"). This statement assume a device, item, etc., that one "uses" is functional, i.e., "works" correctly, but is trying to determine if the device is "easy" to use (understand, operate, etc.) or not easy to use, i.e., difficult to use, complicated, not a user-friendly device, software application, tool, etc.
Note both concepts do not mean the same as (in French) "utile", which in English = "useful". The term "usability" would lean more toward the French term "fonctionnel" and "ease of use" would likely = "facilité d'utilisation" or something along those lines.
it's funny: I live in France but if you were a Russian speaker I could give you "native language" equivalents for these English expressions that would be exact matches; much trickier to accomplish this w/ French, an incredibly "nuanced" language when it comes to "deep meaning" of so many expressions. (Which is why I strive to communicate in French as simply as possible!!!) -- exRanger (edwin)