What happens when we fill User Name and Password fields on a website and click login or submit button?

I am little new to programming (especially to web designing). I have learned that the World Wide Web is based upon a protocol called HTTP. And also each and every item (I mean web pages, images, css & js files etc) are transferred according to the HTTP Requests. So my problem is this.

1. When we fill a web form (especially a login form like fb) and click ok, login or submit button, What Happens Next? Does it send another http request or does it use some special technique?

2. Is it safe or does anyone can hack our user names and passwords when that requests are traveling through internet?