http vs https

We have exchange server running OWA(Outlook Web Access). Till last week we able to access our mails from internet through our weblink. (Ex:https://webapp.example.com/exchange). But now we can only access mail from internet http://webapp.exanple.com/exchange. What is the difference between http and https?. Is it related to 128 bit encryption security. How to check our digital certificate or 128bit encreption expired? Please help me.