If you still want to know this... there is a difference of degree.
Let's say you wanted to know if you could get a particular sort of lucky hand in a card game twice in a row. Since it could happen by some remote chance, it's possible, but the student of probability will tell you it's not probable--that is, it's unlikely.
Here's an article on this difference: http://www.dailywritingtips.com/probable-vs-possible/
So, you're working on a highly dangerous top-secret thing for the government. Your coworker says, "Well, this could either end up working spectacularly, or it could end up blowing up in our faces." You reply, "Well, what's likely?" Although, "which one" might fit better, "what" still works.
It should never be accepted in this sentence.
German doesn't make a distinction between predicative adjectives and adverbs -- they both use the bare adjective stem. But English does.
So Er kommt wahrscheinlich später = He will probably come later. But Das ist wahrscheinlich = That is probable. (Not: That is probably.)
Or even: Das ist wahrscheinlich richtig. = That is probably correct. (wahrscheinlich as adverb, modifying richtig) versus Das ist wahrscheinlich. = That is probably (wahrscheinlich as adjective).