Improbable
Part of speech: adjective
Definition: Not likely to be true. Not likely to happen.
Example sentence: The world is divided into two classes, those who believe the incredible, and those who do the improbable.
Unlikely
Part of speech: adverb
Definition: In an improbable manner.
Part of speech: adjective
Definition: Not likely; improbable; not to be reasonably expected; as, an unlikely event; the thing you mention is very unlikely. Not holding out a prospect of success; likely to fail; unpromising; as, unlikely means.
Example sentence: There is a rumour going around that I have found God. I think this is unlikely because I have enough difficulty finding my keys, and there is empirical evidence that they exist.