Mythos, the greek word, originally meant "History". The mythology of a culture originated as a means of recording what that culture thought to be the real and actual facts about the origin and existence of the cosmos around them.
However, over time the word took up other meanings. When a new religion overthrew the previous one, the old one myths became legends and began being regarded as false, or as fantasy but with no true historical value. That's when the word Mythos became synonim for "fairy tale".
So, to put it in a single sentence, a myth is a belief --often religious-- that has lost its popularity and credibility.
Religions, on the other hand, we can consider to be active and valid beliefs in a given culture. Nowadays, Christianity and Islam, to name just two, are called religions because they are regarded as truths by entire cultures. But when time passes, they will both end being just collections of myths, and will be replaced by newer religions, just as the Egyptian and Norse religions became myths after Islam and Christianity took hold in the geographic areas where they existed before.