What technology is used to create websites that allow users to "sign in?"?

How exactly are websites, like Fluther ,created so that users can sign in. I’m creating a site where I’ll be showing video tutorials. In order to watch the tutorials I would like my site’s visitors to sign in. I would also like to give them the option of leaving comments on the videos. I’m still learning Web design, and my site still needs a lot of work, but I’d like to get a good idea on what I have to look forward to. Thanks dudes.

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