Back in the good old days, the "dark web" was kind of a generic term for sites that didn't have domain names. So, for example, instead of going to gowilkes.com, you might go to https://123.45.67.89 (a made-up IP just for this example). That let sites operate under the radar, without search engines finding them; you just had to know the IP to type in.
I think that's called the "deep web" now.
These days, you need a special browser to anonymously view the dark web. Beyond that, it's not much different.
Is it legal? Technically yes, but the wide majority of the sites on the dark web aren't exactly on the up and up! And there's really no reason why someone would use it unless they're wanting to do something illegal.
For the most part, you would never use it unless you know exactly what you're doing. Think of it like a flea market, but where every vendor just got out of prison and is trying to find a way to rob you.
