The short answer is:

Steemit is a website based on the steem blockchain.
If you post anything with, the content will be saved in the blockchain.
Every site (dtube, dlive) post content into the steem blockchain with a few tags to filter it.
Steemit is a nice website to show every content in steem (That contains everything, the raw content and content with tags, like dtube etc.).
But that's a really short version, if you really want to know how steem works, you can find really good descriptions about that.
I'm not an expert and I hope that helps a little bit. (:

https://steemit.com/steemit/@panenka/what-is-steemit-and-how-does-it-work