HTTPS, which stands for Hypertext Transfer Protocol Secure , is basically an upgraded version of HTTP. Under the hood, the main mechanisms that make it work are kind of the same; but instead of using its own layer protocol for sending data, HTTPS uses additional protocols, secure ones, called a Secure Sockets Layer (SSL), and Transport Layer Security (TLS). SSL encrypts the information that's being sent, which means that even if someone intercepts it, they won't be able to decode it anyway. The true meaning of the data is not visible without the encryption/decryption key.
So, how do you know if a website has HTTPS? Finding this out is very simple. Basically, your web browser will let you know. When you're visiting a website that has HTTPS enabled (and properly integrated), you'll see a green padlock icon next to the website's address. Additionally, you should also see https:// in the address bar. The following screenshot shows an example of this from PayPal:
