Data is encrypted when you access a HTTPS site, but not with HTTP. This means a third party can view your connection to the site, and any information submitted by you.

The connection to HTTP can be exploited, and malicious script can be injected to the response you (the user) receive from the server. This can include ads that drain computer resources (memory, CPU usage), or attempts to exploit vulnerabilities within your operating system/browser. As there is no authentication used with HTTP, there is no way for the user to know if the request/response has been changed.

It is considered generally safe to just browse these sites without inputting sensitive data, but the above is something to be aware of.

Two things come to mind:

• bad guys being able to see what you’re doing
• bad guys sending you something else when you make a request to the website

Both of these cases fall under the category of what’s known as “man in the middle attacks”.

When you visit a site vía HTTPS, it’s using something called Transport Layer Security (TLS). The website encrypts and signs the data coming from it, and your browser verifies that the site is who they say they are and ensures that the data wasn’t tampered with.

Let’s say you visited a site with HTTP (that is, no TLS)… an attacker can sit between you and the real website. They can masquerade as the website and when you go to download that application to your machine, can serve malware to you instead. They also have the opportunity to read anything you’re seeing/doing on the website.

If you’re just going to read some random article and the site is http, there’s unlikely to be any real consequences. These days though, there really isn’t any reason not to use TLS, it’s just so easy.

That the traffic is intercepted and anything that you type in such as passwords or credit cards is stolen, that the content you see is discovered by a third party (ie. Someone finds out you've been reading X specific content on the website) or that the data is altered in flight and injected with false links or other false information.

It's usually no big deal to visit an "unsecure website" for informational purposes if you don't type in any sensitive info.

Generally speaking treat it like a postcard where everything you write could be read or altered by "mail employees" or other third parties.